Animals need sufficient space to ensure physical health and to permit the performance of important behaviors. The effects of spatial restriction will increase with duration of confinement. In addition, diurnal rhythms are relevant. Animals may often require only sufficient space to rest comfortably but, at certain periods, additional space will be required for the performance of comfort, foraging, social or exploratory behavior. The spatial needs of unassuming animals such as chickens and rodents have been assessed using many approaches. First, animals can be housed at relevant allowances and a broad range of measures of health, behavior, physiological status and immune function taken to assess overall welfare. Second, the behavior of animals can be observed after spatial restriction is removed. A significant rebound in behavior performance would indicate that this behavior was being prevented by confinement. Third, the strength of animals' choices for additional space can be examined. Evidence from all three approaches showed that the extreme spatial confinement experienced by laying hens in conventional battery cages resulted in poor welfare, helping to secure a phase-out of conventional cages in Europe by 2012. It has been more difficult to establish optimum space allowances for species kept under less extreme restriction, such as laboratory mice and rats. No consistent effects on welfare were found when inbred and outbred mice were housed at 60, 100 or 167 cm2/mouse. In contrast, sleep disruption was increased when young rats housed were housed at higher stocking densities. Finally, assessment of optimum spatial allowance (stocking density) for animals housed in large groups, is complicated by complex interactions between enclosure size and group number.