The pre-metastatic niche -the accumulation of aberrant immune cells and extracellular-matrix proteins in target organs primes the initially healthy organ microenvironment and renders it amenable for subsequent colonization by metastatic cancer cells. By attracting metastatic cells, mimics of the pre-metastatic niche offer both diagnostic and therapeutic potential. However, deconstructing the complexity of the niche by identifying the interactions between cell populations as well as the mediatory roles of the immune system, soluble factors, extracellular-matrix proteins and stromal cells has proved challenging. Experimental models are needed to recapitulate niche-population biology in situ and to mediate in vivo tumour-cell homing, colonization and proliferation. In this Review, we outline the biology of the pre-metastatic niche and discuss advances in the engineering of niche-mimicking biomaterials that regulate the behaviour of tumour cells at an implant site. Such 'oncomaterials' offer strategies for the early detection of metastatic events, the inhibition of the formation of the pre-metastatic niche and the attenuation of metastatic progression.