Russian is the most geographically widespread language of Eurasia, the most widely spoken of the Slavic languages, and the largest native language in Europe. Russian belongs to the family of Indo-European languages and is one of three living members of the East Slavic languages, the others being Belarusian and Ukrainian Written examples of Old East Slavonic are attested from the 10th century onwards. Today, Russian is widely used outside Russia. Over a quarter of the world's scientific literature is published in Russian. Russian is also a necessary accessory of world communications systems (broadcasts, air- and space communication, etc). Because of the status of the Soviet Union as a superpower, Russian had great political importance in the 20th century and was widely taught in primary and higher education as a foreign language in many countries all over the world. Hence, the language is one of the six official languages of the United Nations.

Russian distinguishes between consonant phonemes with palatal secondary articulation and those without, the so-called soft and hard sounds. This distinction is found between pairs of almost all consonants and is one of the most distinguishing features of the language. Another important aspect is the reduction of unstressed vowels, which is somewhat similar to that of English. Stress, which is unpredictable, is not normally indicated orthographically though, according to the Russian Language Institute of the Russian Academy of Sciences, an optional acute accent may, and sometimes should, be used to mark stress

Russian is a Slavic language in the Indo-European family. From the point of view of the spoken language, its closest relatives are Ukrainian and Belarusian, the other two national languages in the East Slavic group. In many places in eastern Ukraine and Belarus, these languages are spoken interchangeably, and in certain areas traditional bilingualism resulted in language mixture, e.g. Surzhyk in eastern Ukraine and Trasianka in Belarus. An East Slavic Old Novgorod dialect, although vanished during the fifteenth or sixteenth century, is sometimes considered to have played a significant role in the formation of the modern Russian language.

The vocabulary (mainly abstract and literary words), principles of word formations, and, to some extent, inflections and literary style of Russian have been also influenced by Church Slavonic, a developed and partly adopted form of the South Slavic Old Church Slavonic language used by the Russian Orthodox Church. However, the East Slavic forms have tended to be used exclusively in the various dialects that are experiencing a rapid decline. In some cases, both the East Slavic and the Church Slavonic forms are in use, with many different meanings. For details, see Russian phonology and History of the Russian language.

Russian phonology and syntax have also been influenced to some extent by the numerous Finnic languages of the Finno-Ugric subfamily: Merya, Moksha, Muromian, the language of the Meshchera, Veps, et cetera. These languages, some of them now extinct, used to be spoken in the center and in the north of what is now the European part of Russia. They came in contact with Eastern Slavic as far back as the early Middle Ages and eventually served as substratum for the modern Russian language. The Russian dialects spoken north, north-east and north-west of Moscow have a considerable number of words of Finno-Ugric origin. Over the course of centuries, the vocabulary and literary style of Russian have also been influenced by Western/Central European languages such as Polish, Latin, Dutch, German, French, and English.

According to the Defense Language Institute in Monterey, California, Russian is classified as a level III language in terms of learning difficulty for native English speakers, requiring approximately 780 hours of immersion instruction to achieve intermediate fluency. It is also regarded by the United States Intelligence Community as a "hard target" language, due to both its difficulty to master for English speakers as well as due to its critical role in American world policy.

The Russian language is primarily spoken in Russia, Ukraine, Kazakhstan and Belarus, and, to a lesser extent, the other countries that were once constituent republics of the USSR. During the Soviet period, the policy toward the languages of the various other ethnic groups fluctuated in practice. Though each of the constituent republics had its own official language, the unifying role and superior status was reserved for Russian. Following the break-up of the USSR in 1991, several of the newly independent states have encouraged their native languages, which has partly reversed the privileged status of Russian, though its role as the language of post-Soviet national discourse throughout the region has continued.
Ethnographic map of the Slavic peoples prepared by Czech ethnographer Lubor Niederle showing territorial boundaries of Slavic languages in Eastern Europe in the mid 1920s

In Latvia its official recognition and legality in the classroom have been a topic of considerable debate in a country where more than one-third of the population is Russian-speaking (see Russians in Latvia). Similarly, in Estonia, Russophones constitute 25.6% of the country's current population and 58.6% of the native Estonian population is also able to speak Russian. In all, 67.8% of Estonia's population can speak Russian. Command of Russian language however is rapidly decreasing among younger Estonians (primarily being replaced by the command of English). For example, if 53% of Ethnic Estonians between 15-19 claim to speak some Russian, then among the 10-14 year old group, command of Russian has fallen to 19% In Kazakhstan and Kyrgyzstan, Russian remains a co-official language with Kazakh and Kyrgyz respectively. Large Russian-speaking communities still exist in northern Kazakhstan, and ethnic Russians comprise 25.6% of Kazakhstan's population.

Those who speak Russian as a mother or secondary language in Lithuania have represented approximately 60% of the population of Lithuania. Also, more than half of the population of the Baltic states speak Russian either as foreign language or as mother tongue. As the Grand Duchy of Finland was part of the Russian Empire from 1809 to 1918, and a number of Russian speakers have remained in Finland, there are 33,400 Russian speakers in Finland, amounting to 0.6% of the population. 5000 (0.1%) of them are late 19th century and 20th century immigrants or their descendants, and the rest are recent immigrants, who have arrived in the 1990s and later.

In the twentieth century, Russian was widely taught in the schools of the members of the old Warsaw Pact and in other countries that used to be allies of the USSR. In particular, these countries include Poland, Bulgaria, the Czech Republic, Slovakia, Hungary, Albania and Cuba. However, younger generations are usually not fluent in it, because Russian is no longer mandatory in the school system. According to the Eurobarometer 2005 survey, though, fluency in Russian remains fairly high (20-40%) in some countries, in particular those where the people speak a Slavic language and thereby have an edge in learning Russian (namely, Poland, Czech Republic, Slovakia, and Bulgaria). It is currently the most widely-taught foreign language in Mongolia and has been compulsory in Year 7 onward as a second foreign language since 2006.

Russian is also spoken in Israel by at least 750,000 ethnic Jewish immigrants from the former Soviet Union (1999 census). The Israeli press and websites regularly publish material in Russian. Russian is also spoken as a second language by a small number of people in Afghanistan (Awde and Sarwan, 2003). According to a BBC report from October, 2009, Afghan refugee children are learning Russian in school. If they return to Afghanistan, this may create a small population of second-language Russian speakers there as well.

Sizable Russian-speaking communities also exist in North America, especially in large urban centers of the U.S. and Canada such as New York City, Philadelphia, Boston, Los Angeles, Nashville, San Francisco, Seattle, Toronto, Baltimore, Miami, Chicago, Denver and the Cleveland suburb of Richmond Heights. In a number of locations they issue their own newspapers, and live in ethnic enclaves (especially the generation of immigrants who started arriving in the early sixties). Only about a quarter of them are ethnic Russians, however. Before the dissolution of the Soviet Union, the overwhelming majority of Russophones in North America were Russian-speaking Jews. Afterwards, the influx from the countries of the former Soviet Union changed the statistics somewhat. According to the United States 2000 Census, Russian is the primary language spoken in the homes of over 700,000 individuals living in the United States.

Significant Russian-speaking groups also exist in Western Europe. These have been fed by several waves of immigrants since the beginning of the twentieth century, each with its own flavor of language. Germany, the United Kingdom, Spain, Portugal, France, Italy, Belgium, Greece, Brazil, Norway, Austria and Turkey have significant Russian-speaking communities totaling 3 million people. Australian cities Melbourne and Sydney also have Russian speaking populations, with the most Russians living in south-east Melbourne, particularly the suburbs of Carnegie and Caulfield. Two thirds of them are actually Russian-speaking descendants of Germans, Greeks, Jews, Azerbaijanis, Armenians or Ukrainians, who either repatriated after the USSR collapsed or are just looking for temporary employment.

Judging by the historical records, by approximately 1000 AD the predominant ethnic group over much of modern European Russia, Ukraine and Belarus was the Eastern branch of the Slavs, speaking a closely related group of dialects. The political unification of this region into Kievan Rus' in about 880, from which modern Russia, Ukraine and Belarus trace their origins, established Old East Slavic as a literary and commercial language. It was soon followed by the adoption of Christianity in 988 and the introduction of the South Slavic Old Church Slavonic as the liturgical and official language. Borrowings and calques from Byzantine Greek began to enter the Old East Slavic and spoken dialects at this time, which in their turn modified the Old Church Slavonic as well.

Dialectal differentiation accelerated after the breakup of Kievan Rus' in approximately 1100. On the territories of modern Belarus and Ukraine emerged Ruthenian and in modern Russia medieval Russian. They definitely became distinct since the 13th century, i.e following the division of that land between the Grand Duchy of Lithuania, Poland and Hungary in the west and independent Novgorod and Pskov feudal republics plus numerous small duchies (which came to be vassals of the Tatars) in the east.

The official language in Moscow and Novgorod, and later, in the growing Muscovy, was Church Slavonic which evolved from Old Church Slavonic and remained the literary language for centuries, until the Petrine age, when its usage shrank drastically to biblical and liturgical texts. Russian developed under a strong influence of the Church Slavonic until the close of the seventeenth century; the influence reversed afterwards, leading to corruption of liturgical texts.

The political reforms of Peter the Great (Пётр Вели́кий, Pyótr Velíkiy) were accompanied by a reform of the alphabet, and achieved their goal of secularization and Westernization. Blocks of specialized vocabulary were adopted from the languages of Western Europe. By 1800, a significant portion of the gentry spoke French, less often German, on an everyday basis. Many Russian novels of the 19th century, e.g. Lev Tolstoy's War and Peace, contain entire paragraphs and even pages in French with no translation given, with an assumption that educated readers won't need one.

The modern literary language is usually considered to date from the time of Aleksandr Pushkin in the first third of the nineteenth century. Pushkin revolutionized Russian literature by rejecting archaic grammar and vocabulary (so called "высо́кий штиль" — "high style") in favor of grammar and vocabulary found in the spoken language of the time. Even modern readers of younger age may only experience slight difficulties understanding some words in Pushkin’s texts, since only few words used by Pushkin became archaic or changed meaning. On the other hand, many expressions used by Russian writers of the early 19th century, in particular Pushkin, Mikhail Lermontov (Михаи́л Ле́рмонтов), Nikolai Gogol (Никола́й Го́голь), Alexandr Griboyedov , became proverbs or sayings which can be frequently found even in the modern Russian colloquial speech.