Therapeutic drug delivery and diagnostics systems comprise biologically active compounds associated with particulate carriers of less than 20 nm. These systems can be utilised for targeted modification of growth, development and functions, such as gene expression, protein synthesis, intracellular energy production and transport mechanisms in prokaryotic and eukaryotic organisms. The systems are also applicable for controlled modification of structural and functional properties of extracellular components and tissue constituents. The characteristics of a biological site are evaluated and an entity is provided which is dependent on the site characteristics. The entity comprises nanoparticles of less than 20 nm. A probe comprising nanoparticles of less than 5 nm is also provided.