Homeostasis. The health of an organism is influenced by external and internal changes that may lead to the loss of homeostasis. Under healthy conditions organisms compensate these changes. If compensation fails disease ensues. Attention will be paid to lifestyle, environmental changes, genetic makeup and health system. It will be answered how lifestyle, environment, genetic makeup and social conditions help to maintain or upset the biological balance and lead to cancer. Tumor formation. To understand this process the transfer of intracellular and the pathways of extracellular information (signal transduction) will be reviewed briefly. Loss of cellular balance may lead to cell death (.e.g. apoptosis) or to rapid cell growth of cells leading to tumor formation. Metastasis. Animal tumor models serve to understand the spread of the primary tumor cells to distant locations of the organism. Different types of tumors and metastases will be reviewed.