The provision of straw in animal production systems is widely presumed to be beneficial for the welfare of the animals. The aim of this paper is to review the scientific basis of this assumption for pigs and cattle. As there are important disadvantages (cost, labour, hygiene and incompatibility with manure drainage systems) associated with the use of straw, studies investigating whether there are suitable alternatives to straw that fulfil the same welfare functions are also reviewed. It is concluded that straw has many positive effects on the welfare of pigs. Bedding improves the physical comfort of the floor, and-unless temperatures are high-straw enables pigs to somewhat control their microclimate thereby increasing thermal comfort. Straw also functions as an important stimulus and outlet for exploration, foraging, rooting and chewing behaviours. Pigs that are feed restricted or housed in barren environments, in particular, can be strongly motivated to express these behaviours and the inability to do so may result in behavioural problems or anomalies. In addition, it has been demonstrated that preparturient sows are highly motivated to obtain nesting material and that straw can have a beneficial effect on maternal behaviour after farrowing. Although there may be superior alternatives for each of these functions of straw separately, it remains unlikely that these alternatives can adequately replace the total combination of these functions and also offer advantages regarding hygiene, environment, labour and economics. The importance of straw for the welfare of cattle mainly concerns floor-comfort. However, it appears that the provisioning of (high quality) synthetic lying mats, perhaps in combination with soft walking floors, may provide floor-comfort equal to that of straw. Although the consumption of straw reduces feeding motivation, and hence, the development of oral stereotypies, the behavioural function of straw is less for cattle compared to pigs. Moreover, it is possible to compose more appropriate roughage-feeds that better fulfil the behavioural as well as the dietary needs of cattle. For both pigs and cattle, there is weak evidence that concrete flooring rather than straw is a risk factor for increased overall morbidity and mortality. However, the relation between straw and health is complex, equivocal and disease specific. (c) 2005 Elsevier B.V. All rights reserved.