An innovative methodology — idiographic clinical trials (ICTs) — is introduced as a way to inform randomized controlled trials (RCTs) in terms of RCT planning (eg, sample size, effect size), use in research scenarios when RCTs are not feasible (eg, rare diseases with small populations), or use in applied settings such as clinical practice, where RCT parameters cannot be followed. ICTs can be conducted generally for lower cost with faster completion time than RCTs. ICTs should not be seen as replacements for RCTs, but as a way to help inform RCTs or provide insights for early product development without allocating the resources for an RCT for early evaluation of an asset. The term idiographic clinical trials comes from its emphasis on within-individual processes over time. Compared to RCTs, this approach is adept for early phase clinical trials, pilot studies, and testing whether efficacy from an RCT can be replicated in a specific clinical setting or subpopulation (eg, patients with RCT exclusion criteria). ICTs couple two well-known methodologies to yield rigorous results from small samples: subject-as-owncontrol experimental designs (eg, crossover designs) with hierarchical linear modeling (or multi-level modeling) refined specifically for small samples.