The promise of administrative data is that they are quickly available, clean, large, without missing data, and longitudinal. This case study discusses all these promises and downsizes the expectations of register data and situates the real assets of registers for scientific research. The conclusion is that register data are outstanding datasets for longitudinal research, but they come at a price: (1) A lengthy application period reduces the immediate usability of the data. (2) The complex data structure requires intermediate or even advanced data management skills. (3) The longitudinal character and the size of the data require careful inspection and preparation of the data before they can be analyzed. But if you look beyond these pitfalls, you will find yourself with rich data, obtained without interviewer bias and collected over a substantial period of time. It allows studying particular populations, often small in size, with sufficient statistical power to make interferences about them.