Background Pelvic organ prolapse is a common problem in women. About 40% of women will experience prolapse in their lifetime, with the proportion expected to rise in line with an ageing population. Women experience a variety of troublesome symptoms as a consequence of prolapse, including a feeling of 'something coming down' into the vagina, pain, urinary symptoms, bowel symptoms and sexual difficulties. Treatment for prolapse includes surgery, pelvic floor muscle training (PFMT) and vaginal pessaries. Vaginal pessaries are passive mechanical devices designed to support the vagina and hold the prolapsed organs back in the anatomically correct position. The most commonly used pessaries are made from polyvinyl-chloride, polythene, silicone or latex. Pessaries are frequently used by clinicians with high numbers of clinicians offering a pessary as first-line treatment for prolapse. This is an update of a Cochrane Review first published in 2003 and last published in 2013. Objectives To assess the effects of pessaries (mechanical devices) for managing pelvic organ prolapse in women; and summarise the principal findings of relevant economic evaluations of this intervention. Search methods We searched the Cochrane Incontinence Specialised Register which contains trials identified from the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, MEDLINE In-Process, MEDLINE Epub Ahead of Print, ClinicalTrials.gov, WHO ICTRP and handsearching of journals and conference proceedings (searched 28 January 2020). We searched the reference lists of relevant articles and contacted the authors of included studies. Selection criteria We included randomised and quasi-randomised controlled trials which included a pessary for pelvic organ prolapse in at least one arm of the study. Data collection and analysis Two review authors independently assessed abstracts, extracted data, assessed risk of bias and carried out GRADE assessments with arbitration from a third review author if necessary. Main results We included four studies involving a total of 478 women with various stages of prolapse, all of which took place in high-income countries. In one trial, only six of the 113 recruited women consented to random assignment to an intervention and no data are available for those six women. We could not perform any meta-analysis because each of the trials addressed a different comparison. None of the trials reported data about perceived resolution of prolapse symptoms or about psychological outcome measures. All studies reported data about perceived improvement of prolapse symptoms. Generally, the trials were at high risk of performance bias, due to lack of blinding, and low risk of selection bias. We downgraded the certainty of evidence for imprecision resulting from the low numbers of women participating in the trials. Pessary versus no treatment: at 12 months' follow-up, we are uncertain about the effect of pessaries compared with no treatment on perceived improvement of prolapse symptoms (mean difference (MD) in questionnaire scores -0.03, 95% confidence interval (CI) -0.61 to 0.55; 27 women; 1 study; very low-certainty evidence), and cure or improvement of sexual problems (MD -0.29, 95% CI -1.67 to 1.09; 27 women; 1 study; very low-certainty evidence). In this comparison we did not find any evidence relating to prolapse-specific quality of life or to the number of women experiencing adverse events (abnormal vaginal bleeding or de novo voiding difficulty). Pessary versus pelvic floor muscle training (PFMT): at 12 months' follow-up, we are uncertain if there is a difference between pessaries and PFMT in terms of women's perceived improvement in prolapse symptoms (MD -9.60, 95% CI -22.53 to 3.33; 137 women; low-certainty evidence), prolapse-specific quality of life (MD -3.30, 95% CI -8.70 to 15.30; 1 study; 116 women; low-certainty evidence), or cure or improvement of sexual problems (MD -2.30, 95% -5.20 to 0.60; 1 study; 48 women; low-certainty evidence). Pessaries may result in a large increase in risk of adverse events compared with PFMT (RR 75.25, 95% CI 4.70 to 1205.45; 1 study; 97 women; low-certainty evidence). Adverse events included increased vaginal discharge, and/or increased urinary incontinence and/or erosion or irritation of the vaginal walls. Pessary plus PFMT versus PFMT alone: at 12 months' follow-up, pessary plus PFMT probably leads to more women perceiving improvement in their prolapse symptoms compared with PFMT alone (RR 2.15, 95% CI 1.58 to 2.94; 1 study; 260 women; moderate-certainty evidence). At 12 months' follow-up, pessary plus PFMT probably improves women's prolapse-specific quality of life compared with PFMT alone (median (interquartile range (IQR)) POPIQ score: pessary plus PFMT 0.3 (0 to 22.2); 132 women; PFMT only 8.9 (0 to 64.9); 128 women; P = 0.02; moderate-certainty evidence). Pessary plus PFMT may slightly increase the risk of abnormal vaginal bleeding compared with PFMT alone (RR 2.18, 95% CI 0.69 to 6.91; 1 study; 260 women; low-certainty evidence). The evidence is uncertain if pessary plus PFMT has any effect on the risk of de novo voiding difficulty compared with PFMT alone (RR 1.32, 95% CI 0.54 to 3.19; 1 study; 189 women; low-certainty evidence). Authors' conclusions We are uncertain if pessaries improve pelvic organ prolapse symptoms for women compared with no treatment or PFMT but pessaries in addition to PFMT probably improve women's pelvic organ prolapse symptoms and prolapse-specific quality of life. However, there may be an increased risk of adverse events with pessaries compared to PFMT. Future trials should recruit adequate numbers of women and measure clinically important outcomes such as prolapse specific quality of life and resolution of prolapse symptoms. The review found two relevant economic evaluations. Of these, one assessed the cost-effectiveness of pessary treatment, expectant management and surgical procedures, and the other compared pessary treatment to PFMT.