Within the information systems research, there is a long tradition for longitudinal research, and it plays a significant role in the research literature. In this chapter, we will overview the reasons provided by researchers for when a longitudinal study is appropriate. Longitudinal studies have a particular focus on time and change. Time and change address a concern for understanding the details of human actors’ behaviour and perceptions both as individuals and in social arrangements. This addresses ‘how’ to conduct a longitudinal study and why a deeper level of understanding is beneficial. In this chapter, we will map longitudinal research in information systems from the last two decades. This mapping shows critical distinctions that can be used in designing longitudinal research. The most important difference in longitudinal studies is between variance studies and process studies. Variance studies set the research design before the data collection, treat the change over time as a black box, favour a positivist stance and ask what-questions to see how the input causes the output over time. Process studies have a research design that emerges gradually as the data collection and analysis moves forward, favours an interpretive stance and asks what happens within the process.