Although cycling is a fundamental method of transportation throughout the Netherlands, it is particularly important in Leiden due to the high student population and the relatively small size of the community. There is a massive parking lot for bicycles at the Leiden Central Train Station (Leiden Centraal).
If you are planning to spend an extended period of time in the Netherlands, it is certainly worth purchasing a bicycle. Otherwise, it is possible to rent a bike on either an hourly, daily or weekly basis.
