In recent years, the study of terrorism and violent extremism has increasingly benefited from the use of register data such as population data and judicial data. The use of this type of data brings with it many advantages, such as the ability to study complete populations, to combine different sources of data, and, often, it enables researchers to take on a longitudinal and comparative approach. However, the use of register data also brings with it some significant challenges, in terms of acquiring the data (practical challenges) and in terms of biases in the data (content/ethical challenges). In this paper we discuss the usage of register data in terrorism research, with a focus on the Dutch and Swedish context. Whereas both countries are to some extent similar in that they both have a rich data-infrastructure, differences also exist regarding the type of data, agencies responsible for collecting and distributing the data, and ethical approaches. In the chapter, we will go into the strengths and weaknesses of using register data in research on terrorism and violent extremism. Moreover, we will elaborate on how the use of register data can be improved, what type of research questions can and cannot be answered based on register data, and on potential opportunities of integrating register data into mixed-methods designs.