Tumour necrosis factor alpha is a member of the TNF/TNFR cytokine superfamily. In common with other family members, TNF-α is involved in maintenance and homeostasis of the immune system, inflammation and host defence. However, there is a ‘dark side’ to this powerful cytokine; it is now clear that, especially in middle and old age, TNF-α is involved in pathological processes such as chronic inflammation, autoimmunity and, in apparent contradiction to its name, malignant disease. This article will discuss the involvement of TNF-α in the inflammatory network that contributes to all stages of the malignant process, and consider the possibility that TNF-α may be a target for cancer therapy.