Heritage languages (HL) are language spoken by the children of immigrants or by those who immigrated to a country when young. The purpose of this article is to briefly review what is known about heritage language development over time and to identify some gaps in people's knowledge. In this article, the authors consider three aspects: how much HL speakers use their HLs, how well they know them, and the attitudes they have toward their HLs, focusing here on older children, adolescents, and adults. (Contains 4 tables.)