question: BACKGROUND: Pressure ulcers (also known as pressure injuries) are localised injuries to the skin or underlying soft tissue, or both, caused by unrelieved pressure, shear or friction. Foam surfaces (beds, mattresses or overlays) are widely used with the aim of preventing pressure ulcers. OBJECTIVES: To assess the effects of foam beds, mattresses or overlays compared with any support surface on the incidence of pressure ulcers in any population in any setting. SEARCH METHODS: In November 2019, we searched the Cochrane Wounds Specialised Register; the Cochrane Central Register of Controlled Trials (CENTRAL); Ovid MEDLINE (including In‐Process & Other Non‐Indexed Citations); Ovid Embase and EBSCO CINAHL Plus. We also searched clinical trials registries for ongoing and unpublished studies, and scanned reference lists of relevant included studies as well as reviews, meta‐analyses and health technology reports to identify additional studies. There were no restrictions with respect to language, date of publication or study setting. SELECTION CRITERIA: We included randomised controlled trials that allocated participants of any age to foam beds, mattresses or overlays. Comparators were any beds, mattresses or overlays. DATA COLLECTION AND ANALYSIS: At least two review authors independently assessed studies using predetermined inclusion criteria. We carried out data extraction, 'Risk of bias' assessment using the Cochrane 'Risk of bias' tool, and the certainty of the evidence assessment according to Grading of Recommendations, Assessment, Development and Evaluations methodology. If a foam surface was compared with surfaces that were not clearly specified, then the included study was recorded and described but not considered further in any data analyses. MAIN RESULTS: We included 29 studies (9566 participants) in the review. Most studies were small (median study sample size: 101 participants). The average age of participants ranged from 47.0 to 85.3 years (median: 76.0 years). Participants were mainly from acute care settings. We analysed data for seven comparisons in the review: foam surfaces compared with: (1) alternating pressure air surfaces, (2) reactive air surfaces, (3) reactive fibre surfaces, (4) reactive gel surfaces, (5) reactive foam and gel surfaces, (6) reactive water surfaces, and (7) another type of foam surface. Of the 29 included studies, 17 (58.6%) presented findings which were considered at high overall risk of bias. Primary outcome: pressure ulcer incidence Low‐certainty evidence suggests that foam surfaces may increase the risk of developing new pressure ulcers compared with (1) alternating pressure (active) air surfaces (risk ratio (RR) 1.59, 95% confidence interval (CI) 0.86 to 2.95; I(2) = 63%; 4 studies, 2247 participants), and (2) reactive air surfaces (RR 2.40, 95% CI 1.04 to 5.54; I(2) = 25%; 4 studies, 229 participants). We are uncertain regarding the difference in pressure ulcer incidence in people treated with foam surfaces and the following surfaces: (1) reactive fibre surfaces (1 study, 68 participants); (2) reactive gel surfaces (1 study, 135 participants); (3) reactive gel and foam surfaces (1 study, 91 participants); and (4) another type of foam surface (6 studies, 733 participants). These had very low‐certainty evidence. Included studies have data on time to pressure ulcer development for two comparisons. When time to ulcer development is considered using hazard ratios, the difference in the risk of having new pressure ulcers, over 90 days' follow‐up, between foam surfaces and alternating pressure air surfaces is uncertain (2 studies, 2105 participants; very low‐certainty evidence). Two further studies comparing different types of foam surfaces also reported time‐to‐event data, suggesting that viscoelastic foam surfaces with a density of 40 to 60 kg/m(3) may decrease the risk of having new pressure ulcers over 11.5 days' follow‐up compared with foam surfaces with a density of 33 kg/m(3) (1 study, 62 participants); and solid foam surfaces may decrease the risk of having new pressure ulcers over one month's follow‐up compared with convoluted foam surfaces (1 study, 84 participants). Both had low‐certainty evidence. There was no analysable data for the comparison of foam surfaces with reactive water surfaces (one study with 117 participants). Secondary outcomes Support‐surface‐associated patient comfort: the review contains data for three comparisons for this outcome. It is uncertain if there is a difference in patient comfort measure between foam surfaces and alternating pressure air surfaces (1 study, 76 participants; very low‐certainty evidence); foam surfaces and reactive air surfaces (1 study, 72 participants; very low‐certainty evidence); and different types of foam surfaces (4 studies, 669 participants; very low‐certainty evidence). All reported adverse events: the review contains data for two comparisons for this outcome. We are uncertain about differences in adverse effects between foam surfaces and alternating pressure (active) air surfaces (3 studies, 2181 participants; very low‐certainty evidence), and between foam surfaces and reactive air surfaces (1 study, 72 participants; very low‐certainty evidence). Health‐related quality of life: only one study reported data on this outcome. It is uncertain if there is a difference (low‐certainty evidence) between foam surfaces and alternating pressure (active) air surfaces in health‐related quality of life measured with two different questionnaires, the EQ‐5D‐5L (267 participants) and the PU‐QoL‐UI (233 participants). Cost‐effectiveness: one study reported trial‐based cost‐effectiveness evaluations. Alternating pressure (active) air surfaces are probably more cost‐effective than foam surfaces in preventing pressure ulcer incidence (2029 participants; moderate‐certainty evidence). AUTHORS' CONCLUSIONS: Current evidence suggests uncertainty about the differences in pressure ulcer incidence, patient comfort, adverse events and health‐related quality of life between using foam surfaces and other surfaces (reactive fibre surfaces, reactive gel surfaces, reactive foam and gel surfaces, or reactive water surfaces). Foam surfaces may increase pressure ulcer incidence compared with alternating pressure (active) air surfaces and reactive air surfaces. Alternating pressure (active) air surfaces are probably more cost‐effective than foam surfaces in preventing new pressure ulcers. Future research in this area should consider evaluation of the most important support surfaces from the perspective of decision‐makers. Time‐to‐event outcomes, careful assessment of adverse events and trial‐level cost‐effectiveness evaluation should be considered in future studies. Trials should be designed to minimise the risk of detection bias; for example, by using digital photography and by blinding adjudicators of the photographs to group allocation. Further review using network meta‐analysis will add to the findings reported here.