This chapter focuses on four key social movement theories that are useful for understanding the emergence and development of non-governmental organisations (NGOs) are resource mobilisation, framing theory, political opportunity structures/political processes and ‘new’ social movement theory/identity-oriented approaches. Resource mobilisation theory is probably the most applicable of traditional social movement theories for understanding the emergence and behaviour of NGOs. Political opportunities theory is a broad body of work which argues that political conditions for the emergence and development of social movements matter, but the theory is also applicable to NGOs. New social movement theory has occupied itself with trying to explain the then ‘new’ movements of the 1960s and 1970s. Societal trends affect the emergence, development and workings of NGOs. Moreover, globalisation processes and funding cuts affect the workings and operational budgets of NGOs. Also central to the new social movements literature is the concept of collective identity, which refers to the sense of we-ness among activists.