The World Health Organization estimates that 7.6 million people died of cancer in 2005 and 84 million people will die in the next 10 years if action is not taken. More than 70% of all cancer deaths occur in low- and middle-income countries, where resources available for prevention, diagnosis and treatment of cancer are limited or nonexistent. Yet cancer is to a large extent avoidable. Over 40% of all cancers can be prevented. Some of the most common cancers are curable if detected early and treated. Even with late cancer, the suffering of patients can be relieved with good palliative care. Cancer control: knowledge into action, WHO guide for effective programmes is a series of six modules offering guidance on all important aspects of effective cancer control planning and implementation. This fourth module on Diagnosis and treatment shows how to implement effective cancer diagnosis and treatment programmes with a public health approach, within the context of a national cancer control programme.