A bicycle trailer is a motorless wheeled frame with a hitch system for transporting cargo by bicycle. It can greatly increase a bike's cargo capacity, allowing point-to-point haulage of objects up to 3 cubic metres (3000 liters, or 4 cubic yards) in volume that weigh as much as 500 kg. However, very heavily loaded trailers may pose a danger to the cyclist and others, and the voluntary European standard EN 15918 therefore suggests a maximum load of 60 kg on trailers without brakes.

Different types of trailer are designed for various purposes, cargo requirements and riding conditions:

Most trailers have a separate axle for each wheel, like those used on a bicycle. These separate axles usually mount directly on the frame using either threaded nuts, a quick-release mechanism, or some press fit arrangement. The use of separate axles for two-wheel trailers allows the load carrying area to be between the wheels with its base below the axles, so as to keep the centre of gravity relatively low.

Some trailers support a normal axle on two sides, others mount the wheel off one side with a stub axle (a one sided axle).

If included, the fender (or just a mud flap) helps to protect the cargo and the towing bicycle from road spray and dirt. On heavy-duty trailers, the fender may be designed to be capable of bearing heavy loads.

There are various types, from home made to those supplied by the trailer manufacturers.

Hitch positions:

Single-wheel trailers generally use a special frame hitch which attaches to both sides of the rear axle, and which incorporates a vertical hinge to allow cornering. Attaching the trailer at the seat-post can (dependent on the design) place the load at a lower point relative to the longitudinal pivot axis of the attached trailer. This can make it possible to wheel a loaded single-wheel trailer around while disconnected from the bike, as the seat-post hitch is a natural height for grasping while walking upright.