Over the past 10 years, there have been major advances in the understanding of cancer cachexia and asthenia. These common complications of cancer are now thought to be the consequences of complex interactions between host, tumour, and psychosocial factors. Cachexia and asthenia commonly coexist, but they can occur independently of each other. Recently identified tumour-derived factors cause lipolysis and protein catabolism. Cytokines produced by the host in response to tumour presence cause metabolic abnormalities, which result in decreased protein and lipid synthesis, increased lipolysis, and anorexia. Many other factors contribute to asthenia, such as anaemia, autonomic failure, and muscular abnormalities. Future research should clarify optimum management. The way forward seems to lie in a multidimensional approach with combined therapy to manage both cancer cachexia and asthenia.