This chapter introduces the dynamics of ecosystems and chaotic systems, providing an accessible overview for readers unfamiliar with complexity theory. Key concepts such as fractals and emergence are defined and applied to social groups through the FLINT model of Factional Leadership, Intergroup Conflict, Norms, and Time, which explains how factions and subgroups form and ferment within a seemingly unified group. This model examines forces driving subgroup differentiation and the challenges of achieving lasting social change because of the need to influence multiple groups simultaneously and overcome resistance. The chapter revisits psychological research on effective activism, underscoring the importance of addressing both conformity and dissent within and between groups. Finally, we discuss empirical methods for analysing these complex dynamics, including network analyses, person-centred analyses, and agent-based modelling, which offer new ways to understand and study the formation and evolution of groups.