This paper reviews a wide range of literature applicable to understanding why and how hobbyists learn. Of particular importance appear to be theories such as situated learning and communities of practice, but insights from the cognitive sciences related to expertise, motivation, and interest also emerged as important. The boundaries between formal and informal education continue to breakdown, making the need to understand and address the needs of learners of all ages, across multiple settings and situations more important than ever. Learning is becoming increasingly "on-demand", mediated by "colleagues" and initiated in order to solve learner-defined needs and interests (as opposed to the historical condition of prescribed, sequential curricula defined by nameless, faceless experts). Hobbyists afford learning researchers important opportunities to build a better understanding of interest-driven, persistent, contextually relevant, lifelong learning.