Hyaluronan is an extracellular and cell-surface-associated polysaccharide that is traditionally regarded as a biological 'goo' that participates in lubricating joints or holding together gel-like connective tissues. Although these are common physiological roles of hyaluronan in adult organisms, hyaluronan also functions as a microenvironmental cue that co-regulates cell behaviour during embryonic development, healing processes, inflammation and tumour development. Recent work highlights a key role for interactions between hyaluronan and tumour cells in several aspects of malignancy and indicates the possibility of new therapeutic strategies.