Motivation is a fundamental educational challenge for adolescents in the United States, and particularly for immigrant youth. The motivation to achieve, especially in mathematics, declines during adolescence. Most of what is known about motivation is based on studies of predominately middle-class White students; yet the influx of immigrants over the past forty years, particularly from Latin America and Asia, speaks to the increasing importance of achievement among immigrant youth in the U.S. school system and society at large, underscoring the need to understand the motivation to achieve among immigrant youth and its relation to academic achievement. This dissertation examines the association between motivation (i.e., personal goal orientations of mastery, performance-approach, and performance-avoidance) and mathematics achievement among approximately 2,000 7th and 8th grade students, predominantly of Mexican and Vietnamese heritage, from one low-income urban school district in California. A lagged dependent variable regression model with classroom fixed effects is used to minimize biases arising from non-random assignment of teachers and students to schools and classrooms. Study 1 examines the association between students' goal orientations and achievement, as measured by a state standardized assessment. Although all three goal orientations correlate with mathematics achievement, only a mastery goal orientation consistently predicts achievement after controlling for a full set of prior achievement and demographic controls. Study 2 adds nuance to the relationship between goal orientations and achievement with a focus on ethnicity and gender. First, I examine the predictors of the three goal orientations with a focus on mastery motivation. Ethnicity is not associated with mastery motivation--Vietnamese students do not have a motivation advantage over Hispanic students. Gender is associated with mastery motivation, with males less likely than females to be mastery oriented. Additionally, males--particularly Hispanic males--are more performance-avoidant than Hispanic females. Second, the differential effects of goal orientations for students with varying socio-demographic characteristics suggest that the relationship between goal orientations and mathematics achievement are not the same for all students. There is a positive association between mastery goal orientation and achievement for Vietnamese students but not Hispanic students; however, no differential effects are found with performance goals.