Aiming to encourage longitudinal studies in science education, we clarify conceptual and methodological aspects of longitudinal research. We use the studies that other articles in this issue describe to illustrate these aspects. The illustrations range from attempts to promote long-term change through experimental teaching to investigations that describe change without intending to affect it. For both experimental and descriptive studies we discuss the validity of conclusions and insights, and how threats to validity may be reduced or eliminated. We highlight implications for teaching and the curriculum, and for the conduct of research. We describe the practicalities of longitudinal research, referring to time, resources, management of data, and attrition of subjects, and end with a summary of reasons why longitudinal studies are valuable.