Technical Abstract: Egg production systems have become subject to heightened levels of scrutiny due to animal welfare concerns. Multiple factors such as disease, skeletal and foot health, pest and parasite load, behavior, stress, affective states, nutrition, and genetics influence the level of welfare laying hens experience. Although the need to evaluate the influence these factors is recognized, research into these areas is still in the early stages. We compare conventional cages, furnished cages, non-cage systems, and outdoor systems. Attributes of each system are shown to impact welfare and those systems which have similar attributes are impacted similarly. For instance, environments, such as non-cage and outdoor systems, in which hens are exposed to litter and dirt provide a greater opportunity for disease and parasites. The more complex the environment the more difficult it is to clean, and the larger the group size the more easily disease and parasites are able to spread. Environments, such as conventional cages, which limit movement, can lead to osteoporosis; but environments which have increased complexity, such as non-cage systems, expose hens to an increase incidence of bone fractures. More space allows for hens to perform a greater repertoire of behaviors, although some deleterious behaviors such as cannibalism and crowding, which results in smothering, can occur. Less is understood about the stress which each system imposes on the hen, but it appears that each system has its unique challenges. Selective breeding for desired traits such as improved bone strength and decreased feather pecking and cannibalism may help to improve the welfare of laying hens. It appears that no single housing system is ideal from a hen welfare perspective. Although environmental complexity increases behavioral opportunities, it also introduces difficulties in terms of disease and pest control. In addition, environmental complexity creates opportunities for the hens to express behaviors that are actually detrimental to their welfare. As a result, any attempt to evaluate the sustainability of a switch to an alternative housing system requires careful consideration of the merits and shortcomings of each housing system.