This chapter serves as a basis for understanding concepts and theory in the field of energy transitions. Fundamental ideas that shape the subject and its analysis are examined, including early influences. Commonalities and differences between energy transition and sustainability transition writing are also considered. Disruptions, such as net-zero systems, geopolitics, digitalization, and the COVID-19 pandemic are then examined more fully in the context of current energy systems. The chapter closes with takeaways to consider for research, decisions, and informed action.