The above quotation illustrates the importance of identity. It is hard to imagine any behavior a person could engage in that would somehow not have implications for how they see themselves and how the world sees them. Indeed, the question of “Who am I?” is one that we as human organisms ponder. We surmise, re-evaluate and update our self-conceptions throughout our lifespan. The cognitive sophistication and complex ability to articulate self-reflective thoughts separates humans from other species; the ability to define who we are and what we want to become. Therefore, “identity is important” is probably not a controversial statement. That is the easy part. What is more difficult is to pinpoint the best way to define and study it. After all, if something defies definition and measurement, then it is nothing more than lofty philosophical rhetoric, a useful metaphor, perhaps (Cohen 1989). If the idea of “identity” is a serious area of empirical inquiry, then one must face the difficult challenge of developing a precise theoretical, methodological and substantive set of ideas to capture this construct. In that regard, there has been great progress, yet there is much more work to do. From the early days of personality research (Allport 1937; Murray 1938; Barenbaum and Winter 2008), the idea of a monolithic self was appealing. A person has a “self-concept”: the sum total of thoughts, ideas and beliefs about who they are, and what they want to be. If you could identify what the key elements were, then you might be able to predict what that person is going to say, think and do. It made sense to focus on static and enduring traits that may be an important set of building blocks to base this conception on (Cristal and Tupes 1992; Costa and McCrae 1997; Costa et al. 1998). The fact that measuring these traits often produced weak relationships to other outcomes pointed to the need for additional nuance within the conception of what a self-concept actually means (see Griffith and Jenkins 2004). It was the information processing revolution, the self as an organizing structure in memory (Kihlstrom and Klein 1986), and the idea that the self-concept is better thought of as categories or a collection of “social identities” (Tajfel and Turner 1979; Abrams and Hogg 1990) that opened a door forward to deeper understanding. This need for complexity comes at a price, though. Now the questions are: What are the key social categories that matter? When do they matter? Why do they matter? How do they change over time?