Much of social psychology is concerned with processes that play a role in human behaviour, which we take here to be, if not fully synonymous with agency, nonetheless a core aspect of its defining features. This chapter provides a systematic review of how social psychological constructs and related agentic theory have been used to address questions relevant to individual agency in the context of sociotechnical sustainability transitions. Our motivating premise is that an understanding of higher-level transitions processes is incomplete without an understanding of why people act as they do. We find that light, simultaneous reference to social psychology and sociotechnical transitions remains sizeable and is ongoing. The social psychology-related approaches used can be categorised as: (i) variable based psychology; (ii) information, framing and risk; (iii) social practice, culture and lifestyle; (iv) representations and expectations; (v) reviews, syntheses and systems models; (vi) values, norms and traits; (vii) place attachment; (viii) societal and organizational-level concepts. We discuss related ontological issues and conclude that social psychological-related approaches may usefully complement sociotechnical, co-evolutionary approaches to sustainability transitions, with varying degrees of integration being possible.