Good clinical practice (GCP) is a series of systematically developed ethical and quality standard of designing, registering, running, recording, and reporting of the clinical trials. Good clinical practice is very important regarding the trials usually performed on the vulnerable populations especially children and adolescents. The sensitivity of the issue is even higher in the children with psychiatric disorders. Usually, these children have little legal protection. Hence, the safety of interventions and the ethical considerations are among the most important issues in this field. The purpose of this chapter is to deal with above problems and globally applicable standards for the conduct of clinical trials on the under legal age subjects especially those with psychiatric disorders. Selection of trial subjects, ethical principles, regulatory requirements, protection of trial subjects, monitoring (compliance with the protocol), responsibilities of the investigator, and other requirements to perform a clinically and ethically sound clinical trial in children and adolescents will be discussed in this chapter.