Concepts in tumour immunology are changing fundamentally. Around 1970 tumour immunology contained the following related concepts: 1. Thousands of tumour cells arise de novo each day. 2. Tumour cells are antigenic in their host. 3. All these antigenic tumour cells are killed by a strong immune surveillance system. A more likely set of concepts looks as follows: 1. Tumour cells do not arise frequently. 2. Tumour cells may be antigenic or not. 3. There is no need to postulate a very strong immune surveillance or natural resistance system. In this paper I am reviewing our present knowledge of immune surveillance and natural resistance. Only scanty information appears to be available. This information suggests that virally induced tumours are usually killed by cytotoxic T lymphocytes, and natural killer cells, whereas immune surveillance and natural resistance against other tumours may be quite weak.