Drugs usually have no natural affinity for the cells, tissues and organs where therapeutic effects are needed, which frequently results in low efficiency and unwanted side effects. This concern is even more profound when using highly potent and cytotoxic anticancer drugs or specific agents, such as enzymes and genetic materials, since their effective and safe action requires precise cellular or even sub-cellular addressing in the target organ. To meet safety, efficiency and specificity requirements, drugs somehow must be targeted to the sites of their expected therapeutic action. The idea of the "magic bullet," or drug targeting, proposed by Erlich a century ago, generates great and continuously growing interest in biomedical, industrial and financial circles. This book is focused on the strategies designed to target therapeutic or diagnostic agents to the disease sites. In an attempt to include in this volume the set of chapters reflecting both traditional and emerging areas of drug targeting, we have contacted many leading scientists in the field asking for their contributions. Their responses were most favorable and encouraging. As a result, we have succeeded in assembling a series of outstanding contributions reflecting practically all the key areas of drug targeting. The final structure of this book is as follows.