This study presents a synthesis of the empirical research on identity across science, technology, engineering, and mathematics (STEM) published between 2000-2018. The research canvassed in this study encompasses a range of contexts and ages. A total of 202 articles were examined in order to determine how identity was emphasized and operationalized within each field and by grade/age bands, as well as the multitude of factors shaping individual's identity within STEM. Results confirmed the complex nature of identity and the internal (e.g., confidence) and external factors (e.g., institutional norms) positively and/or negatively shaping youths' and adults' identity within STEM. These factors are distinguishing and perpetuating the in-group, or those who identify as a member of a STEM field, from the outgroup, or those who do not identify as a member of a STEM field. Additionally, the findings of this study highlighted identity in the field of technology, and with adult participants, as among the least investigated fields and grade/age bands respectively. Conversely, we have a more holistic picture of science and mathematics identities across age/grade bands. Other findings demonstrated an overemphasis on representational identities in comparison to identities as narratives and identities as ways of acting. The findings generated from this review serve in providing research directions concerning identity studies within the STEM field.