Most interventions for basal cell carcinoma (BCC) have not been compared in head-to-head randomized trials. To evaluate the comparative effectiveness and safety of treatments of primary BCC in adults. English-language searches of MEDLINE, Cochrane Central Register of Controlled Trials, Cochrane Database of Systematic Reviews, and Embase from inception to May 2018; reference lists of guidelines and systematic reviews; and a search of ClinicalTrials.gov in August 2016. Comparative studies of treatments currently used in adults with primary BCC. One investigator extracted data on recurrence, histologic clearance, clinical clearance, cosmetic outcomes, quality of life, and mortality, and a second reviewer verified extractions. Several investigators evaluated risk of bias for each study. Forty randomized trials and 5 nonrandomized studies compared 18 interventions in 9 categories. Relative intervention effects and mean outcome frequencies were estimated using frequentist network meta-analyses. Estimated recurrence rates were similar for excision (3.8% [95% CI, 1.5% to 9.5%]), Mohs surgery (3.8% [CI, 0.7% to 18.2%]), curettage and diathermy (6.9% [CI, 0.9% to 36.6%]), and external-beam radiation (3.5% [CI, 0.7% to 16.8%]). Recurrence rates were higher for cryotherapy (22.3% [CI, 10.2% to 42.0%]), curettage and cryotherapy (19.9% [CI, 4.6% to 56.1%]), 5-fluorouracil (18.8% [CI, 10.1% to 32.5%]), imiquimod (14.1% [CI, 5.4% to 32.4%]), and photodynamic therapy using methyl-aminolevulinic acid (18.8% [CI, 10.1% to 32.5%]) or aminolevulinic acid (16.6% [CI, 7.5% to 32.8%]). The proportion of patients reporting good or better cosmetic outcomes was better for photodynamic therapy using methyl-aminolevulinic acid (93.8% [CI, 79.2% to 98.3%]) or aminolevulinic acid (95.8% [CI, 84.2% to 99.0%]) than for excision (77.8% [CI, 44.8% to 93.8%]) or cryotherapy (51.1% [CI, 15.8% to 85.4%]). Data on quality of life and mortality were too sparse for quantitative synthesis. Data are sparse, and effect estimates are imprecise and informed by indirect comparisons. Surgical treatments and external-beam radiation have low recurrence rates for the treatment of low-risk BCC, but substantial uncertainty exists about their comparative effectiveness versus other treatments. Gaps remain regarding high-risk BCC subtypes and important outcomes, including costs. Agency for Healthcare Research and Quality. (PROSPERO: CRD42016043353).