The goal of pharmaceutical scientists has long been to deliver drugs to specific target tissues at the right amount, at and for the right amount of time and with a minimum of side effects. Although these goals remain to a large extent unachieved for many drugs, the tools available with which to attack these goals have become increasingly sophisticated over recent years and offer the promise of truly novel approaches to targeted and controlled drug delivery. These novel approaches presented by emerging new therapeutic modalities, such as those based on nanotechnology and biotechnology, also present increasing challenges to be overcome by the pharmaceutical chemist. Biotechnology products for example may be antigenic, have very short half-lives, or may not distribute effectively to the target tissue compartment. Or perhaps gene therapy products may need to be targeted to specific cells within a single tissue to avoid inappropriate modification of nontarget cells, and as such their delivery mechanism requires the capacity for very precise target differentiation.