Fuel may be designed for fission gas containment within the fuel clad or for venting, also to survive possible ruptures of the clad. Fission gas behavior varies widely with the UO2 starting material and, like the baking of bread, with its irradiation history of time and temperature. Below 1000° C, most gas remains trapped; between 1000 and 1800° C, most gas becomes trapped at some time in small closed pores. Expansion of the pores causes swelling. Escape from the pores is by cracking or by radiation-induced reabsorption. Fuel rods are commonly designed with a plenum to collect gas, but the high density of pores and plasticity of the oxide delays or prevents much of the gas from reaching the plenum. For irraditions up to 12 000 MWd/t, satisfactory designs have been achieved without a plenum. Above 1800° C, most gas escapes, but the process of escape is not well established and requires consideration in each case by the design engineer. Available knowledge most relevant to design is briefly reviewed and illustrated in photographs and graphs.