Little empirical research has examined the cause of language shift or the factors related to the heritage language (HL) development of language minority groups. This study investigates the factors that facilitate or inhibit the HL development of second-generation Korean Americans, with a focus on adolescent HL learners, who go through phases in the development of their identity, ethnic identity, and their attitudes toward HL development and HL speakers, which are shaped during these phases. Most of the U.S.-born Korean-American high school students who participated in this study were English dominant and preferred to speak in English. The majority of participants had positive attitudes toward maintaining or developing their HL and wanted to develop their HL to have a closer relationship with their family, to be able to communicate with Korean speakers, to understand Korean culture, and to expand their career opportunities. Findings also showed, however, that HL maintenance or development is a challenging task in the U.S., and that language shift to the dominant language of the country was evident. The following areas of recommendations are discussed: (1) Maximize Comprehensible HL Input; (2) Minimize Anxiety; (3) Develop Good HL Programs; (4) Provide a Variety of HL Reading Materials; and (5) Maximize Informal Language Learning. The results of this study can be used for planning educational programs that take into account the needs and resources of immigrant families, as well as for informing the public, preservice teachers, and educators about the value of maintaining and developing one's HL.