Publisher Summary To understand drug actions, it is necessary to consider the effects produced by the drug on the biological system at various levels of complexity of organization. The main levels, from the most complex to the most simple, could be designated as follows: intact organism, organized cells, cells, sub-cellular structures, and biological molecules. The effects produced by a drug can be recognized only as an alteration in a function or process that maintains the existence of the living organism, because all drugs act by producing changes in some known physiological function or process. The functional macromolecular structures, which are targets for drug action, could be arbitrarily divided into receptors, enzymes, proteins involved in the transport, and nucleic acids. A number of other molecular interactions between drugs and the components of the biological system may occur, such as the binding of drugs to plasma albumin or other constituents of the tissues. Serum albumin can transport drugs in the circulation to various organs, and it can hold drugs up, preventing them from binding to their site of action. Those interactions have secondary, rather than primary, consequences for pharmacodynamic actions, because the duration of the drug action or its rate of actions is affected. Albumin might then be considered as an acceptor site for the drug rather than a receptor.