This chapter introduces clinical research concepts and randomised clinical trials, covering the basics and building blocks that are necessary to understand the topics of adaptive trial designs and master protocols. Clinical trials are a type of prospective experimental studies in which human volunteers receive specific interventions according to the research protocol, then are followed longitudinally over time. Clinical trials are typically conducted in a sequence (from phase I, phase IIA, phase IIB, and phase III) that builds on knowledge accumulated from non-clinical and previous clinical studies. Randomisation is a process of random assignment of clinical trial participants to one or more intervention group(s) or control group under comparison. The use of randomisation provides a sound basis for making statistical causal inference when estimating the comparative treatment effects between groups. Fixed sample trial design refers to a type of designs where the trial data is only analysed once when a priori determined sample size has been reached. Fixed sample trial designs are designed with a fixed maximum sample size, a fixed number of interventions, and a defined end to the trial. This is the most common approach to clinical trial research.