A second language or L2 is any language learned after the first language or mother tongue. Some languages, often called auxiliary languages, are used primarily as second languages or lingua francas (such as Esperanto).

A person's first language may not be their dominant language, the one they use most or are most comfortable with. For example, the Canadian census defines first language for its purposes as "the first language learned in childhood and still spoken", recognizing that for some, the earliest language may be lost, a process known as language attrition. This can happen when young children move, with or without their family (because of immigration or international adoption), to a new language environment.

According to some researchers, the defining difference between a first language (L1) and a second language (L2) is the age the person learned the language. For example, linguist Eric Lenneberg used second language to mean a language consciously acquired or used by its speaker after puberty. In most cases, people never achieve the same level of fluency and comprehension in their second languages as in their first language. These views are closely associated with the Critical Period Hypothesis.

Stephen Krashen is professor emeritus at the University of Southern California, who moved from the linguistics department to the faculty of the School of Education in 1994. He is a linguist, educational researcher, and activist.

Dr. Krashen has published more than 350 papers and books, contributing to the fields of second language acquisition, bilingual education, and reading. He is credited with introducing various influential concepts and terms in the study of second language acquisition, including the acquisition-learning hypothesis, the input hypothesis, the monitor hypothesis, the affective filter, and the natural order hypothesis. Most recently, Krashen promotes the use of free voluntary reading during second language acquisition, which he says "is the most powerful tool we have in language education, first and second."

As education policy in Krashen’s home state of California became increasingly hostile to bilingualism, he responded with research critical of the new policies, public speaking engagements, and with letters written to newspaper editors. During the campaign to enact an anti-bilingual education law in California in 1998, known as Proposition 227, Krashen campaigned aggressively in public forums, media talk shows, and conducted numerous interviews with journalists writing on the subject. After other anti-bilingual education campaigns and attempts to enact regressive language education policies surfaced around the country, by 2006 it was estimated that Krashen had submitted well over 1,000 letters to editors.