The assessment of risks associated with engineered nanomaterials demands a careful adaptation of existing risk assessment procedures. In principle, the procedures established to evaluate the risk of conventional substances are also applicable to nanomaterials. However, their specific properties and behaviour need adequate consideration. As engineered nanomaterials comprise a diverse and relatively young group of substances, risk evaluators need to deal with data gaps and act on a case-by-case basis. This chapter aims to give an overview on the present and future challenges in risk assessment of engineered nanomaterials.