Cycling is cheaper, healthier and, in urban environments, often faster than other modes of transport. Nevertheless, many individuals do not cycle even for short distances. This publication aims to explain why commuters differ in their decisions as to whether or not to cycle. Results indicate that the individual (day-to-day) choice to commute by bicycle is affected by personal attitudes towards cycling to work, social norms, the work situation, weather conditions and trip characteristics. In addition, the book provides evidence that different groups of bicycle commuters exist: non-cyclists, part-time cyclists and full-time cyclists. The mode choice of individuals within these groups depends partly on a number of different factors. Non-cyclists seem not to cycle because they consider it impractical, either due to the distance involved, their need to transport goods, the need for a car during office hours or a negative subjective norm. The decision to cycle among part-time or full-time cyclists is also affected by these factors, but additional factors can be identified. Finally, the day-to-day choice to cycle is based on work characteristics, weather conditions and trip characteristics. Part-time cyclists who cycle only occasionally are encouraged by pleasant weather conditions, while frequent cyclists are found to be discouraged by more practical barriers, such as where they need to work on a particular day.