BACKGROUND: An inguinal hernia occurs when part of the intestine protrudes through the abdominal muscles. In adults, this common condition is much more likely in men than in women. Inguinal hernia can be monitored by 'watchful waiting', but if symptoms persist or worsen, surgery is usually required, which can be open or laparoscopic. Laparoscopic (keyhole) repair of inguinal hernias in adults is generally performed using either the transabdominal preperitoneal (TAPP) or the totally extraperitoneal (TEP) method. Both methods include the use of mesh placed in front of the peritoneal lining of the abdominal wall, but for the TAPP technique, the abdominal cavity needs to be entered to place the mesh, and for the TEP technique, the whole procedure is done on the outside of the peritoneal lining of the abdominall wall. Whether one method is superior to the other has not been established, and there is debate about their relative benefits and harms. An advantage of TEP is its avoidance of the abdominal cavity; the downside is that it requires a steeper learning curve for clinicians. TAPP is considered simpler and makes it possible to inspect the contralateral side, but TAPP may have a higher risk of visceral injury compared to TEP. This is an update of a Cochrane review first published in 2005. OBJECTIVES: To compare the benefits and harms of laparoscopic TAPP technique versus laparoscopic TEP technique for inguinal hernia repair in adults. SEARCH METHODS: On 25 October 2022, the authors searched the Cochrane Central Register of Controlled Trials (CENTRAL) in the Cochrane Library; Ovid MEDLINE(R) Epub Ahead of Print, In-Process & Other Non-Indexed Citations, Ovid MEDLINE(R) Daily, and Ovid MEDLINE(R); and Ovid Embase, for published randomised controlled trials. To identify studies in progress, we searched ClinicalTrials.gov and the WHO International Clinical Trial Registry Platform (ICTRP). SELECTION CRITERIA: All prospective randomised, quasi-randomised, and cluster-randomised trials that compared the laparoscopic TAPP technique with the laparoscopic TEP technique for inguinal hernia repair in adults were eligible for inclusion. We included studies that involved a mix of different types of groin hernia if we could extract data for the inguinal hernias. Studies may have also included a group of participants receiving hernia repair by open surgery, but these groups were not included in our review. DATA COLLECTION AND ANALYSIS: Both review authors independently evaluated trial eligibility, extracted data from included studies, and assessed the risk of bias in the included studies. The review's primary outcomes were serious adverse events, chronic pain (persisting for at least six months after surgery), and hernia recurrence. We also assessed a variety of secondary outcomes at perioperative, early postoperative, and late postoperative time points. We performed statistical analyses using the random-effects model, and expressed the results as odds ratios (ORs) for dichotomous outcomes and mean differences (MDs) for continuous outcomes, with their respective 95% confidence intervals (CIs). We used GRADE to assess the certainty of evidence for key outcomes as high, moderate, low or very low. MAIN RESULTS: We included 23 studies in this review update, which randomised 1156 people to TAPP and 1110 people to TEP, all requiring repair of inguinal hernias. Study sample sizes varied from 40 to 316 participants. The vast majority of study participants were male. We judged most studies to be at 'high' or 'unclear' risk of bias. Our judgements of the certainty of the evidence were low or very low for all outcomes we assessed. There may be little to no difference between TAPP and TEP laparoscopic techniques for serious adverse events (0.4% versus 0.7%; OR 0.58, 95% CI 0.15 to 2.32, P = 0.45, I = 0%; 19 studies, 1735 participants; low certainty of evidence); and hernia recurrence (1.2% versus 1.1%; OR 1.14, 95% CI 0.49 to 2.62, P = 0.97, I = 0%; 17 studies, 1712 participants; low certainty of evidence). The evidence is very uncertain about the effects of TAPP versus TEP techniques on chronic pain (OR 0.62, 95% CI 0.20 to 1.97, P = 0.68, I = 0%; 6 studies, 860 participants; very low certainty of evidence). In terms of secondary outcomes, the evidence is very uncertain for TAPP versus TEP techniques for perioperative visceral and vascular injury (15 studies, 1523 participants; very low certainty of evidence), and for haematoma or seroma during the early (≤ 30 days) postoperative phase (OR 0.86, 95% CI 0.54 to 1.37, P = 0.3861, I = 0%; 15 studies, 1423 participants; very low certainty of evidence). TEP technique may carry a higher risk of conversion to another hernia repair method (either TAPP technique or open surgery) when compared to TAPP (2.5% versus 0.7%; OR 0.28, 95% CI 0.09 to 0.84, P = 0.02, I = 0%; 13 studies, 1178 participants; low certainty of evidence). Only two studies (474 participants) reported quality of life in the late (> 30 days) postoperative phase; overall, there was an improvement in quality of life from the pre- to post-operative assessment, but the evidence suggests little to no difference between the techniques (low certainty of evidence). AUTHORS' CONCLUSIONS: This review update found that there may be little to no difference between the TAPP and TEP techniques for serious adverse events, hernia recurrence, or chronic pain (low- to very-low-certainty evidence). Decisions about which method to use will most likely reflect surgeon and patient preference until high-certainty evidence becomes available. There may be a higher risk of needing to convert from TEP to TAPP or open surgery when compared to the risk of needing to convert from TAPP to open surgery (low-certainty evidence). If surgeons opt for TEP as their standard laparoscopic method, they could consider having a strategy for how to handle the potential need for conversion. This might include proficiency in the TAPP approach or having informed the patient about the risk of conversion to open surgery. For surgeons or surgical departments, the choice of a laparoscopic technique should involve shared decision-making with patients and their families or carers. Future research could focus on patient-reported outcomes, such as quality of life.