Over the past two decades, identity has emerged as a concept framing studies of early career researcher experience. Yet, identity is an amorphous concept, understood and used in a range of ways. This systematic review aimed to unpack the underpinnings of the notion of researcher identity. The final sample consisted of 38 empirical articles published in peer-reviewed journals in the last 20 years. Analyses focused on (a) identifying the dimensions used to define researcher identity, and (b) characterising the meta-theories--the underlying assumptions of the research--in relation to these dimensions. We identified four different stances towards researcher identity (clusters), based on variation on the identity dimensions in relation to the meta-theories. We characterised these as (1) transitioning among identities, (2) balancing identity continuity and change, (3) personal identity development through time and (4) personal and stable identity. These stances incorporate thought-provoking nuances and complex conceptualisations of the notion of researcher identity, for instance, that meta-theory was insufficient to characterise researcher identity stance. The contribution of the study is first to be able to differentiate four characterizations of researcher identity--important given that many studies had not clearly expressed a stance. The second is the potential of the four dimensions to help characterise identity, in past as well as future research--thus a useful tool for those working in this area. Many questions remain, but perhaps the biggest is to what extent and under what conditions is identity a productive notion for understanding early career researcher experience?