<p>This study aimed to systematically review the literature for laboratory studies that evaluated the influence of mono- and polywave light-emitting diode (LED) devices on the degree of conversion (DC) of composite resins and adhesive systems. A search of three electronic databases (PubMed, Web of Science, Scopus) and using Google Scholar was conducted through June 2024 to identify eligible studies that compared monowave and polywave LED devices on the DC of composite resins and adhesive systems. Studies that evaluated DC using indirect methods, material other than composite resin and adhesive system, and missing DC values as mean and standard deviation were excluded. Meta-analysis was performed at a significance level of ≤ 0.05 comparing DC values (mean and standard deviation) produced by LEDs. Heterogeneity was quantified using I² values. 79 potentially relevant studies were identified from online databases, 27 were selected for full-text assessment, and 22 were included in this systematic review. Eighteen of the included studies had a high risk of bias and four had a moderate risk of bias. Polywave LEDs significantly improved the DC of composite resins (<i>p</i> ≤ 0.5) and did not affect on adhesive systems (<i>p</i> = 0.18). Despite the high risk of bias and the heterogeneity of the included studies, the available evidence may support the conclusion that polywave LED devices improve the DC of composite resins. Monowave or polywave LED devices did not affect the DC of adhesive systems.</p>