This study was undertaken to investigate the role of self-efficacy in the career trajectories of women who are currently employed in STEM fields and women who had studied in any of these fields, but either never worked in STEM, or decided to leave at some stage. The assumption was that women remain in STEM careers because of the motivational effect of STEM self-efficacy. In order to do this investigation, two studies were included in a parallel convergent mixed-methods design and two samples were studied. The first sample of 15 women, which included both women in STEM (n = 8) and women who had left STEM (n = 7), were interviewed and invited to talk about their STEM studies and careers. The interviews were conducted according to a semi-structured interview. The second sample, which consisted of 108 participants of whom 88 were actively involved in STEM and 20 had left the field, completed an online survey that contained a biographical section, three self-efficacy scales and an Exploratory Questionnaire (EQ) that covered aspects such as motivation to study and work in STEM and barriers experienced. The three self-efficacy scales used were the General Self-Efficacy Scale (GSES), the New General Self-Efficacy Scale (NGSES) and the Occupational Self-Efficacy Scale (OSES). Bandura's Social Cognitive Theory (SCT) was chosen as the conceptual framework for the study and the development of Social Cognitive Career Theory (SCCT) was described from its inception to its current integrated models of career development, as applied to women in STEM careers. The integrated models show that a combination of self-efficacy and outcome expectations is crucial as a predictor of career success in the STEM fields, which can also be influenced by additional variables, such as career decision making, career and study satisfaction, persistence, contextual support and barriers. The Social Cognitive Theory (SCT) and Social Cognitive Career Theory (SCCT) provided the theoretical framework for themes for the qualitative thematic analysis. A top-down identification of themes was done by using the transcripts of interviews. Self-efficacy, outcome expectations and barriers were among the twelve themes that were identified. The survey data was described and statistically analysed. Descriptive statistics were provided for the selfefficacy scales and biographical information. The STEM and non-STEM groups were compared with a series of contingency tables on biographical information. A t-test was used to compare the self-efficacy scales by STEM status in order to find significant differences. The EQ was subjected to an exploratory principal component analysis (PCA) and 10 factors or components were identified. The factors ranged from motivation, barriers and perceptions about gender to STEM and education. Finally, the factors were compared with the qualitative themes to explore the role of self-efficacy in the careers of STEM and non-STEM women. The contribution made by this study is that it highlights the importance of the sources of selfefficacy in ensuring that women remain in their chosen fields. A frequently under-emphasised aspect is that of the emotional source of self-efficacy, which this study found to be the passion, focus, enjoyment and satisfaction that motivate women to remain in STEM. The relevant literature frequently observes that girls and women do not like STEM subjects and activities. However, the passion and commitment of women witnessed by the researcher while conducting this study counters this observation. Some women do enjoy science and it is by no means a proven fact that a lack of interest in STEM is gendered. Programmes focusing on motivating women to enter and remain in STEM ought to take this particular source of selfefficacy into account. The question is, of course, whether one can create interest, instil passion and make STEM attractive to women. However, this is a separate topic for further study. One of the clear findings of this study relates to the importance of inner-circle support and motivation to enter and remain in STEM. Programmes should find a way to encourage families who are already involved in STEM to include children, and especially girls. The very personal nature of encouragement, motivation and support received from parents and close family members function as a major source of self-efficacy. This calls for a creative approach to motivational programmes in order to make commitment to STEM inclusive. Another point that was emphasised by women in the qualitative sample, as well as in the quantitative results, was the major importance of personal interest in the field of science. In fact, this was even more important than the motivational support provided by close family. In essence, it relates to the passion expressed by women in STEM, but the importance of developing a strong interest in science cannot be overstated. Finally, several of the respondents working either in or outside STEM mentioned the pressures experienced in an attempt to balance family and work responsibilities. Some women manage this successfully, even though they are in STEM careers, while others deal with the problem by leaving STEM. However, one should point out that even in non-STEM careers the pressures and expectations of family life and children exist. Programmes dealing with women in STEM should take this problem very seriously and should assist women in effectively managing and dealing with the combined pressures of family and work.