This research studied the development of self-esteem through adolescence and emerging adulthood. It also analyzed sex differences and the role of family and peers in developmental trends in self-esteem. Data comes from a longitudinal study in which we administered the Rosenberg Self-Esteem Scale to 90 Spanish boys and girls at ages 13, 15, 17, and 21 years. Results showed a linear increase in self-esteem, higher for boys than for girls, during adolescence and emerging adulthood. Initial variability was related to care received from the mother during childhood, whereas the increase in self-esteem throughout adolescence and emerging adulthood was related to peer attachment.