Gastarbeiter are foreign or migrant workers, particularly those who had moved to West Germany between 1955 and 1973, seeking work as part of a formal guest worker program. As a result, guestworkers are generally considered temporary migrants because their residency in the country of immigration is not yet determined to be permanent. Other countries had similar programs: in the Netherlands and Belgium it was called the gastarbeider program; in Sweden, Denmark, Norway and Finland it was called arbetskraftsinvandring (workforce-immigration); and in East Germany such workers were called Vertragsarbeiter. The term that was used during the Nazi era was Fremdarbeiter. However, the latter term had negative connotations, and was no longer used after World War II.

Foreign workers or guest workers are people who work in a country other than one of which they are a citizen. Some foreign workers use a guest worker program in a country with more preferred job prospects than in their home country. Guest workers are often either sent or invited to work outside their home country or have acquired a job before leaving their home country, whereas migrant workers often leave their home country without a specific job in prospect.