The purpose of this chapter is to provide you with definitions and the characteristic features of rapid research. It proposes a threshold for distinguishing rapid research from long-term research, while also problematising distinctions based purely on the duration of the study. An important aspect of the chapter is to help you identify when rapid research is suitable and useful. The chapter also briefly explores the main challenges of carrying rapid research, such as: maintaining a clear theoretical grounding of the research despite time pressures, dealing with the tension between the breadth and depth of data, addressing potential issues with sampling where recruitment might be limited to those who are most accessible, devoting limited time to reflexivity, and addressing potential delays in the management and governance of the research. These challenges are explored in greater detail in the rest of the chapters.