Objective. Randomized controlled trials (RCTs) are often considered the gold standard in evaluating whether intervention results are in line with causal claims of beneficial effects. However, given that poor design and incorrect analysis may lead to biased outcomes, simply employing an RCT is not enough to say an intervention “works.” This paper applies a subset of the Society for Prevention Research (SPR) Standards of Evidence for Efficacy, Effectiveness, and Scale-up Research, with a focus on internal validity (making causal inferences) to determine the degree to which RCTs of preventive interventions are well-designed and analyzed, and whether authors provide a clear description of the methods used to report their study findings. Methods. We conducted a descriptive analysis of 851 RCTs published from 2010-2020 and reviewed by the Blueprints for Healthy Youth Development web-based registry of scientifically-proven and scalable interventions. We used Blueprints’ evaluation criteria that correspond to a subset of SPR’s standards of evidence. Results. Only 22% of the sample satisfied important criteria for minimizing biases that threaten internal validity. Overall, we identified an average of 1-2 methodological weaknesses per RCT. The most frequent sources of bias were problems related to baseline non-equivalence (i.e., differences between conditions at randomization) or differential attrition (i.e., differences between completers versus attritors or differences between study conditions that may compromise the randomization). Additionally, over half the sample (51%) had missing or incomplete tests to rule out these potential sources of bias. Conclusions. Most preventive intervention RCTs need improvement in rigor to permit causal inference claims that an intervention is effective. Researchers also must improve reporting of methods and results to fully assess methodological quality. These advancements will increase the usefulness of preventive interventions by ensuring the credibility and usability of RCT findings.