CNS drug development is rapidly evolving to meet the unique and changing demands of the fields of psychiatry and neurology. Phase I studies are traditionally focused on determining the safety, tolerability, and pharmacokinetics of a new molecular entity in young healthy volunteers: first in a single ascending dose (SAD) study and then in a multiple ascending dose (MAD) study. The use of an adaptive design to examine data and modify the study in real time may also promote time- and cost-efficient identification of novel molecular entities (NMEs) that have the characteristics necessary to be successful in confirm stages of drug development research. This chapter discusses how use of biomarkers as surrogate endpoints can be incorporated into early clinical trials to determine if a drug modulates the appropriate targets, which enhances the likelihood of demonstrating efficacy in the confirm phase of clinical testing.