Nanotechnology, especially nanoparticle research, has played very important roles in many scientific discoveries, laying the foundation for their applications in many aspects of our life, such as healthcare, efficient energy and cleaner environments. The field is moving very quickly, with an increasing number of researchers. This Faraday Discussion focusses on one very fundamental issue of nanoparticle (NP) research: functionalization, relating specifically to biomedical applications. Without the right functionalisation, NPs will be rendered useless. So, how can NPs be synthesised with the desired functionality?