Hoppers are used to transport people between two points. They also create the rivers by hoppering water between moons. In order for a hopper to work, a direct line of sight between the hopper eye and hopper source is needed. Some hoppers work automatically, while others require passwords to be spoken; the hopper on Elboreth works both ways, requiring a password to enter the Catkis but working automatically to let you out. A hopper can also have multiple passwords for different circumstances; some let through robots with any word, others require the speaker to be unarmed, while yet others can be used by anyone to get through. Different passwords can take the speaker to different locations. Most of these passwords are shown to be words of Ancient.
Particles in the air that reflect and refract light can disrupt the signal. The exact effects of this are not known, but are implied to result in the hoppered material being degraded or distorted. It is possible to create hoppers that only partly hopper the subject, for example only hoppering the mind but leaving the body behind. All robots can function as hoppers if a useable eye is available. Robots have been shown to hopper people into locations without hopper eyes in line of sight, but how this is achieved is unkown.
In the past, hoppers behind gates were widely used in lieu of doors with hinges. One such hopper-gate combination can be found on Domed Market Moon. This may be so that only robots and people accompanied by robots could pass, as no-one else would be able to get past the gate.