In this chapter we explore how a modern occupational psychologist can be a scientist-practitioner. We shall approach this discussion from a critical realist perspective (see Chapter 5) and argue that, whilst we as psychological scientistpractitioners have to understand that we operate in a complicated socially constructed world, this knowledge has to be implicitly subsumed into practice rather than dominating it. This means that, in order to act in the world, we must accept this world exists, and matters, outside our own social frameworks. This world is one of industry, commerce and organizations. We shall describe our (unavoidably personal) model of science, provide some examples of practice and stress the notion that, whilst acting in the world requires co-existence within multiple realities, it necessarily means that action which occurs within our clients’ version of reality is the only action that, initially at least, will be perceived by them as legitimate. This interactive and relativist position and our notion of science, we suggest, has its roots in formulations of applied psychology in the last century, particularly in the seminal work of the National Institute of Industrial Psychology (NIIP). It also resonates with more modern notions that espouse a return to a pragmatic model of science in psychology. Thus, we shall examine some of the underlying humanist roots of our profession, the importance of context and relativity in formulating and executing our interventions, and finally, the importance of realism, pragmatism and utility in acting in the world as psychological scientist-practitioners.