In the wild hamsters are highly active at dusk and night and may travel several miles as they forage for food. Therefore it is important to provide the largest cage possible for any pet hamster and the provision of an exercise wheel also helps to provide the hamster with a means to exercise whilst in its cage.
Many of the commercial hamster cages available today have exercise wheels fitted but there are also free standing wheels which can be placed in a cage or aquarium for those that don't.
Any wheel provided should have a solid back and a solid running surface. A hamster can slip on a wheel which has a runged running surface and if it gets its foot caught between the rungs as the wheel is turning it can injure itself. Wheels which have open back/front with a bar across the diameter can also injure the hamster if the hamster should stick its head out whilst running.
Although most hamsters enjoy an exercise wheel, there are a minority who do not and this shouldn't be cause for concern. Some hamsters on the other hand can become quite addicted to their exercise wheels.