Clinical trials are performed to discover or verify the efficacy and safety of one or more investigational medicinal product (IMP). Biological medicinal products, including gene therapies, offer groundbreaking new opportunities for the treatment of disease and injury, but they are also highly regulated and trials with these products can be logistically challenging to set up and execute. To ensure a compliant and successful trial, it is important to know and understand the regulatory framework, and to be aware of available guidance documents published to advise the different stakeholders on how to develop, manufacture, handle, administer, or destroy these products safely and legally. This chapter summarizes the standard requirements and considerations applicable for clinical trials with IMPs and also describes additional requirements for trials with gene therapies or genetically modified microorganisms (GMM).This chapter has been written from the perspective of a UK noncommercial (academic) sponsor. As such, the discussion and guidance has its basis in gene therapy research as governed by UK law. Nevertheless, European legislation and guidance documents are also referenced; most of the following recommendations will be applicable to clinical trials with a gene therapy medicinal product in any European Member State, and the overriding principles would be applicable to any trial.