Although the immune system evolved to protect the host from infection, what fires the popular imagination is its potential to recognise and destroy cancer. The immune system can generate potent cytotoxicity (eg transplant rejection), but can these mechanisms be harnessed for therapeutic benefit in patients with cancer? The discovery of an ever-increasing array of tumour antigens shows clearly that the targets exist. The challenge lies in generating a sufficiently potent response towards them. Central to the processes of antigen recognition, processing, and presentation to the immune system are dendritic cells. Understanding of the relation between these and the cellular immune response is crucial to elucidation of how to manipulate immune responses. The past 20 years have witnessed a dramatic expansion in this understanding and led to the first early-phase clinical trials of dendritic cells for the treatment of cancer. These studies have established the safety and feasibility of this approach and have produced encouraging evidence of therapeutic efficacy. This paper reviews the biology of dendritic cells and their use in clinical trials, as well as highlighting issues for future trial design.