Nanomaterials have a wide range of applications, leading to highly diverse exposure scenarios for humans. This calls for an analysis of how nanoparticles enter the human body and what health hazards they pose there. Current evidence indicates that the smaller the nanoparticles, the more pronounced their toxic effects. Beyond size, however, the shape and chemical composition of nanoparticles contribute to their biological impacts. Nanoparticles are known to induce inflammation in the lung, and some reports of pulmonary fibrosis are available. There are indications that nanoparticles penetrate vascular tissue and therefore trigger certain dysfunctions or influence the cardiovascular system. One recent study involving an animal model demonstrates that needle-shaped, asbestos-like nanotubes can induce chronic inflammations. Only few data are available on effects on the digestive tract and the nervous system or on the uptake into the bloodstream via the skin. This contribution presents the key data and outlines the potential pathways by which nanoparticles can enter the human body.