One place where statisticians can have a major impact is in the design of clinical trials. There are many clinical, regulatory, and statistical considerations that factor into the design of a Phase 3 trial. Should a crossover or parallel trial be used? What endpoints should be considered? Which should be primary and which should be secondary? For treatment of a rare disease, there is also the consideration of minimizing the number of subjects while meeting the regulatory requirements. Crossover trials can reduce the number of subjects required while parallel trials eliminate the chance of any carryover effect. Of course, feedback from regulatory agencies such as the FDA must be accounted for. Design considerations will be discussed using as an example the design of a Phase 3 trial for an acute treatment of a rare disease with episodic attacks. The benefits of crossover vs. parallel design in this setting will be discussed along with sample size and endpoint considerations. Highlights from the FDA draft guidance on rare diseases will be provided.