RATIONALE: This is an update of a Cochrane review first published in 2007. Allergic disease and food allergy are prevalent, and contribute to a significant burden of disease on the individual, their family and the healthcare system. Probiotics are live bacteria that colonise the gastrointestinal tract, and have been studied in many clinical trials for preventing allergic conditions. OBJECTIVES: To evaluate the benefits and harms of a probiotic, or a probiotic with added prebiotic ('synbiotic'), compared with control (placebo or no treatment) for preventing allergic diseases (asthma, eczema, allergic rhinitis) and dietary allergies in infants by two years of age. SEARCH METHODS: We searched CENTRAL, MEDLINE, Embase and trial registries in December 2023. We reviewed the reference lists of studies selected for inclusion in this review, and systematic reviews on similar topics. We manually searched conference abstracts. ELIGIBILITY CRITERIA: We included randomised controlled trials that compared a probiotic to a control, or a probiotic added to a prebiotic ('synbiotic'). We included enterally fed infants in the first six months of life without clinical evidence of allergic disease. We included probiotics added to human milk or infant formula, added in the manufacturing process or given separately. OUTCOMES: Infant incidence by two years of age and childhood incidence (up to 10 years of age or up to the age of latest report between 2 and 10 years) of specific allergic diseases, including: asthma, eczema, allergic rhinitis, immunoglobulin E (IgE)-mediated food allergy, IgE-mediated cow's milk protein allergy. Events of anaphylaxis and potential harms including adverse effects, harms or infection with probiotic bacteria. RISK OF BIAS: We used the Cochrane RoB 2 tool to assess bias in the studies. SYNTHESIS METHODS: We used the random-effects (Mantel-Haenszel) model for meta-analysis where possible. Where this was not possible due to the nature of the data, we synthesised and interpreted individual studies separately. We used GRADE to assess the certainty of evidence for each outcome. INCLUDED STUDIES: We included 24 studies (7077 mother-infant pairs). The studies were conducted in many parts of the world, including the USA, Europe, South Korea, Japan, Singapore and Australia, with most being conducted in Europe. Studies were published between 2001 and 2020. As some studies measured outcomes such as eczema using different criteria, we made assumptions to allow us to combine data. SYNTHESIS OF RESULTS: Probiotics may result in little to no difference in asthma (risk ratio (RR) 0.96, 95% confidence interval (CI) 0.65 to 1.44; 4 studies, 954 participants; low-certainty evidence), allergic rhinitis (RR 0.89, 95% CI 0.45 to 1.77; 5 studies, 1045 participants; low-certainty evidence) and IgE-mediated cow's milk protein allergy (RR 0.99, 95% CI 0.82 to 1.20; 4 studies, 259 participants; low-certainty evidence) by two years of age. Probiotics may result in a slight reduction in eczema by two years of age (RR 0.87, 95% CI 0.78 to 0.97; 18 studies, 3494 participants; low-certainty evidence); however, sensitivity analysis of the studies at low risk of bias showed little or no difference in eczema by two years of age (RR 0.86, 95% CI 0.69 to 1.07; 4 studies, 892 participants). Probiotic supplementation may have little to no effect on the incidence of food allergy by two years, but the evidence is very uncertain (RR 1.12, 95% CI 0.57 to 2.20; 3 studies, 857 participants; very low-certainty evidence). The evidence is very uncertain about the effect of synbiotics on eczema by two years of age (RR 0.88, 95% CI 0.52 to 1.47; 3 studies, 1235 participants; very low-certainty evidence). Synbiotics may result in little to no difference in food allergy by two years of age (RR 1.06, 95% CI 0.55 to 2.07; 1 study, 223 participants; low-certainty evidence). There were no data for the effect of synbiotics on asthma, allergic rhinitis and IgE-mediated cow's milk protein allergy by two years of age. Probiotic or synbiotic supplementation may result in little to no difference in potential harms including adverse effects, harms or infection with probiotic bacteria at any point during the study intervention by two years of age. There were no serious adverse events related to probiotics or synbiotics reported. We had some concerns about risk of bias for most studies, with only a few judged at low risk of bias. Some studies had a high risk of bias due to unclear randomisation, missing data and lack of prespecified intentions. Estimates were often imprecise, with wide CIs due to limited events. The limited data prevented subgroup analyses on infant risk factors and feeding methods for outcomes other than the effect of probiotics on eczema. Only three studies assessed synbiotic supplementation, leaving their role in allergic disease prevention uncertain. The included studies were mainly in high-income countries in many different areas of the world, but may have limited applicability to other regions. AUTHORS' CONCLUSIONS: There is insufficient evidence to make conclusions about the effect of probiotics and synbiotics on preventing the development of allergic diseases by two years of age and during childhood up to 10 years of age. Although there were no serious adverse events reported for the use of probiotics in infants, incorporating probiotics and synbiotics into routine practice requires further information to support their use. FUNDING: This Cochrane review had no dedicated funding. REGISTRATION: Protocol (2007) available via https://doi.org/10.1002/14651858.CD006475. Original review (2007) available via https://doi.org/10.1002/14651858.CD006475.pub2.