Clinical trials are essential to ensure that the drugs, devices and related entities that come into use by the general population are safe and efficacious for utility. The drug development process is a lengthy, regulated and streamlined process that starts in the laboratory, moving into animal studies and then finally into human trials. These human trials, also called clinical trials are divided into 5 phases—0, 1, 2, 3 and 4. Phases 1 to 4 are performed for all drugs and devices (without waivers), while Phase 0 is more of an exploratory trial, which is not routinely conducted. However, Phase 0 is very important in a few therapeutic areas and clinical populations. Phase 0 trials are sometimes interchangeably known as “micro-dosing studies”. The most common application of Phase 0 trials is in the area of oncology. This chapter details the intricacies of Phase 0 trials in full length.