Research suggests that principals exercise a measurable effect on school effectiveness. Although indirect, this effect is significant and supports the view that principal's leadership contributes to student achievement. Achieving expected goals is particularly difficult for schools serving large concentrations of students who are living in poverty, have limited English proficiency, and have persistently low academic achievement. These schools require principals who, in the midst of challenges and intense scrutiny, remain confident in their ability to overcome challenges, set direction, develop capacity, and implement structures that support effective teaching and learning. Self-efficacy beliefs are a key cognitive factor influencing principals' leadership behaviors in complex school environments. The purpose of this research was to investigate the relationship between principals' self-efficacy beliefs and student achievement as measured by gains in API scores. In addition, the study examined the factors that influence principals' self-efficacy perceptions. Elementary school principals serving Title I schools in California were the unit of study. This investigation employed a mixed-method sequential explanatory design. The first, quantitative phase, addressed two research questions: whether principals' self-efficacy predicted gains in API and whether personal and school demographic variables predicted principals' self efficacy beliefs. The qualitative phase sought explanations to the quantitative findings. This research found that principals' self-efficacy perceptions were predictive of gains in API. Among the demographic variables, PI status had a negative effect on principals' self-efficacy beliefs. Principals in the sample had their efficacy beliefs strengthened as a result of performance accomplishment and relationships with mentors. These findings are consistent with theory and research and underscore the importance of considering social cognitive theories in the study of principal leadership. They also suggest important implications for district leaders and others responsible for developing and supporting principals. Implications for further research are discussed.