Considerable research on ego identity has appeared over the past fifteen years, indicating the need for an overall review. Part I commences by considering the complexity of Erikson's concept and suggesting several different theoretical contexts in which it has been used. The validity of investigators' attempts to utilize the concept "ego identity" is assumed to depend in part upon their interpretations of its meaning. The review of empirical literature is organized according to different procedures which have been used to measure ego identity. A first section summarizes and evaluates research utilizing Q-sort and self-report questionnaire measures, while a second considers the large number of investigations that have employed James Marcia's Identity Status Interview. The latter group of studies are ordered according to the type of dependent variable (e.g., cognitive, behavioral, personality, developmental) examined in relation to ego identity status (i.e., achievement, moratorium, foreclosure, diffusion).