Homelessness research faces unique challenges and thus the question of appropriate methods is crucial in order to obtain data at the national as well as at the European level that can be used as a solid foundation for policy development. Typical challenges in homelessness research are related to the nature of the research population and are an ongoing subject of refinement and improvement in order to generate comparable data across countries. Definitions of homelessness on which consequent research is based are closely related to ideological preconceptions on the cause of homelessness as well as to pragmatic methodological decisions. Particular attention in the definition of research populations needs to be paid to vulnerable groups who are potentially at risk to fall into homelessness in order to understand the processes leading to homelessness and to enable the formulation of preventive measures. For risk groups, too, definitions are required.