Background: Delta-like ligand 3 (DLL3), a ligand of the Notch-family, is a potential therapeutic target in small cell lung cancer (SCLC). High expression levels of DLL3 are observed in SCLC, however different methods and thresholds are used. A systematic literature review (SLR) was conducted to estimate the prevalence of DLL3 expression in SCLC. Methods: A comprehensive literature search was conducted in the PubMed, EMBASE, Web of Science, and Cochrane Library databases, as well as on clinicaltrials.gov, using a pre-specified search strategy. This SLR was conducted and reported following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The protocol was registered in the International Prospective Register of Systematic Reviews (CRD42022351119). Prevalence of DLL3 expression was evaluated overall and by method of DLL3 assessment and positivity threshold. Results: Of the 30 studies that met our study inclusion criteria, 22 (73.3%) were observational and 8 (26.7%) were clinical trials. Most studies (N = 21, 70%) evaluated less than 100 patient specimens (range: 20 to 1,362 patient specimens). Twelve studies (40%) were conducted in Asia, six (20%) in Europe, five (17%) in North America, five (17%) were multi-country, one in Brazil, and geographical location was not reported in one study. DLL3 testing methods varied across studies, with immunohistochemistry (IHC) being used in 26 (86.7%) studies. Two studies used real-time polymerase chain reaction, one used quantitative mass spectrometry, and one study did not report the testing method. Thirteen of the studies (13/26, 50%) that evaluated DLL3 expression by IHC used the Ventana SP347 assay. DLL3 expression was evaluated at various thresholds of tumor cell staining and showed a wide range of expression, across studies (Table). Conclusions: The SLR findings indicate that most studies assessing DLL3 in SCLC have relied on IHC methodology. Despite the range of thresholds used for evaluating positivity, DLL3 is notably expressed in SCLC. The frequency and impact of aberrant DLL3 localization was not addressed in these studies and minimal data on comparison of IHC with other methodologies exists. Results of this SLR also highlight the need to define a standardized method and threshold of evaluating DLL3 expression.