The study of volunteerism has generated multiple conceptual frameworks yet no integrated theory has emerged. This article identifies three major challenges, or layers of complexity, that a unified theory of volunteering faces. First, volunteering is a complex phenomenon that has permeable boundaries and spans a wide variety of activities, organizations, and sectors. Second, different disciplines attribute different meanings and functions to volunteering. Third, existing theoretical accounts are biased toward covering the 'laws of volunteering' and have a strong empirical surplus.'Good theory' however is multidimensional so there is a need to include other views on theory. To overcome these challenges, we use a 'hybrid theoretical strategy' that seeks to combine the 'multiple goodness' of current approaches. Our hybrid framework builds on the three layers of complexity identified, and provides an innovative conceptual system of navigation to map, compare, and integrate existing theories more adequately.