A trial design where each participant receives both the real treatment and the comparison treatment in sequence — so every person acts as their own control.Eliminates between-person variability because the same individual experiences both treatments. Requires a washout period between treatments to allow the first compound to clear. Appropriate for stable, reversible conditions. Some SS-31/elamipretide heart failure trials used crossover designs. Not appropriate when treatments have long-lasting or irreversible effects, or when the condition is expected to change over time.