Nanoparticles offer many unexplored possibilities in biomedicine. The controlled delivery of drugs and the direction into malignant or infected tissue in particular will have a bright future when a reliable and selective addressing has been finally accomplished. To achieve this goal, drug-loaded nanoparticles must be developed, and the work of chemists is necessary to attach specialised molecules on to their surface. To assess the biological action, a strong collaboration between bioscientists and physicians is necessary; both to assure the optimal biological action and to exclude possible adverse effects well in advance. The high mobility of nanoparticles in the body due to their small size should not be underestimated. They have the ability to travel into many organs, including the brain. It is therefore essential to pre-empt any adverse effects or accumulations well in advance of a clinical trial. Such investigations are not easy because it is difficult to detect nanoparticles in the body due to their small size. Furthermore, there are still many open questions concerning the fundamental biological action of nanoparticles that need to be elucidated. Many kinds of nanoparticles are known, many have been tested in biological systems, and some are in clinical trials. Our examples come partially from fundamental research and from clinical research, and in some cases; the pathway to a clinical application still has to be travelled.