Inorganic nanoparticles and their accompanying diverse physical properties are now virtually in routine use as imaging tools in cell-biology. In addition to serving as excellent contrast agents, their size- and environment-dependent optical and magnetic properties can be harnessed to create enzyme biosensor devices of extremely high sensitivity, whilst circumventing the numerous technical limitations associated with traditional enzyme assays. This thesis explores new applications of noble metal and semiconductor nanoparticles (quantum dots) for the detection of a range of medically-relevant enzymes. Here, water soluble colloidal gold nanoparticles and quantum dots are modified with peptides and antibodies to afford new reagents for enzyme sensing. These bio-functionalised nanoparticle probes exhibit numerous advantages over traditional enzyme sensing systems owing to their robust and size-tuneable optical properties. These enzyme-responsive nanoparticle systems are tailored to detect the activity of protein kinase and histone acetyltransferase activity in a simple homogeneous assay format based on Förster resonance energy transfer (FRET). Furthermore, these systems can be applied to screen for putative small-molecule modulators of enzyme function. These new assays should provide the basis for the development of a variety of new enzyme detection strategies based on nanoparticlespecific optical phenomena.