Cancer is currently a high-priority area for drug development. Most cancers are immediately life-threatening diseases demanding urgent treatment and therapies are usually highly toxic. This poses a range of specific challenges for the ethical conduct of clinical trials in cancer, including difficulties with performing placebo-controlled studies, blinding, and restricting off-protocol treatments that may impact on trial results. Overall survival is the gold-standard efficacy endpoint for cancer trials, but reliable results can require a long duration of follow-up. Other endpoints such as time to progression and tumour response rates are therefore also used. Where treatments are targeted at specific disease mechanisms, biological endpoints may also be assessed. Safety evaluations require an understanding of the effects of the disease and its treatment on the likely observed events and abnormalities. A thorough understanding of the specifics of the disease under investigation and established as well as experimental approaches to its treatment can help medical writers to produce consistent and accurate documentation throughout clinical development.