Practitioners in the field of assisted reproductive technology (ART) continually seek alternative or adjunct treatments to improve ART outcomes. This Cochrane review investigates the adjunct use of synthetic versions of two naturally produced hormones, dehydroepiandrosterone (DHEA) and testosterone (T), in assisted reproduction. Steroid hormones are proposed to increase conception rates by positively affecting follicular response to gonadotrophin stimulation. This may lead to a greater oocyte yield and, subsequently, an increased chance of pregnancy. To assess the effectiveness and safety of DHEA and T as pre- or co-treatments in infertile women undergoing assisted reproduction. We searched the following electronic databases up to 8 January 2024: the Gynaecology and Fertility Group (CGF) Specialised Register, CENTRAL, MEDLINE, Embase, PsycINFO, and trial registries for ongoing trials. We also searched citation indexes, Web of Science, PubMed, and OpenGrey. We searched the reference lists of relevant studies and contacted experts in the field for any additional trials. There were no language restrictions. Randomised controlled trials (RCTs) comparing DHEA or T as an adjunct treatment to any other active intervention, placebo, or no treatment in women undergoing assisted reproduction. Two review authors independently selected studies, extracted relevant data, and assessed risk of bias. We pooled data from studies using fixed-effect models. We calculated odds ratios (ORs) for each dichotomous outcome. Analyses were stratified by type of treatment. We assessed the certainty of evidence for the main findings using GRADE methods. We included 29 RCTs. There were 1599 women in the intervention group and 1469 in the control group. Apart from three trials, the trial participants were women identified as 'poor responders' to standard in vitro fertilisation (IVF) protocols. The included trials compared either T or DHEA treatment with placebo or no treatment. Pre-treatment with DHEA versus placebo/no treatment: DHEA likely results in little to no difference in live birth/ongoing pregnancy rates (OR 1.30, 95% confidence interval (CI) 0.95 to 1.76; I² = 16%, 9 RCTs, N = 1433, moderate certainty evidence). This suggests that in women with a 12% chance of live birth/ongoing pregnancy with placebo or no treatment, the live birth/ongoing pregnancy rate in women using DHEA will be between 12% and 20%. DHEA likely does not decrease miscarriage rates (OR 0.85, 95% CI 0.53 to 1.37; I² = 0%, 10 RCTs, N =1601, moderate certainty evidence). DHEA likely results in little to no difference in clinical pregnancy rates (OR 1.18, 95% CI 0.93 to 1.49; I² = 0%, 13 RCTs, N = 1886, moderate certainty evidence). This suggests that in women with a 17% chance of clinical pregnancy with placebo or no treatment, the clinical pregnancy rate in women using DHEA will be between 16% and 24%. We are very uncertain about the effect of DHEA on multiple pregnancy (OR 3.05, 95% CI 0.47 to 19.66; 7 RCTs, N = 463, very low certainty evidence). Pre-treatment with T versus placebo/no treatment: T likely improves live birth rates (OR 2.53, 95% CI 1.61 to 3.99; I² = 0%, 8 RCTs, N = 716, moderate certainty evidence). This suggests that in women with a 10% chance of live birth with placebo or no treatment, the live birth rate in women using T will be between 15% and 30%. T likely does not decrease miscarriage rates (OR 1.63, 95% CI 0.76 to 3.51; I² = 0%, 9 RCTs, N = 755, moderate certainty evidence). T likely increases clinical pregnancy rates (OR 2.17, 95% CI 1.54 to 3.06; I² = 0%, 13 RCTs, N = 1152, moderate certainty evidence). This suggests that in women with a 12% chance of clinical pregnancy with placebo or no treatment, the clinical pregnancy rate in women using T will be between 17% and 29%. We are very uncertain about the effect of T on multiple pregnancy (OR 2.56, 95% CI 0.59 to 11.20; 5 RCTs, N = 449, very low certainty evidence). We are uncertain about the effect of T versus oestradiol or T versus oestradiol + oral contraceptive pills. The certainty of the evidence was moderate to very low, the main limitations being lack of blinding in the included trials, inadequate reporting of study methods, and low event and sample sizes in the trials. Data on adverse events were sparse; any reported events were minor. Pre-treatment with T likely improves, and pre-treatment with DHEA likely results in little to no difference, in live birth and clinical pregnancy rates in women undergoing IVF who have been identified as poor responders. DHEA and T probably do not decrease miscarriage rates in women under IVF treatment. The effects of DHEA and T on multiple pregnancy are uncertain. Data regarding adverse events were very limited; any reported events were minor. Research is needed to identify the optimal duration of treatment with T. Future studies should include data collection on adverse events and multiple pregnancy.