Prodrugs are pharmacologically inert and nontoxic chemical compounds which are transformed in vivo into pharmacologically and therapeutically active compounds, i.e. drugs. Prodrugs are used to solve problems which cannot be solved by the drugs themselves. There are three types of problems: problems arising before the drug enters the body (solubilization, stabilization and improved acceptability); problems associated with the penetration and fate of the drug in the body (absorption, barrier crossing, duration of action); problems relating to the therapeutic target (selective local delivery). The principal prodrugs that solve these problems and the mechanisms of the solutions they provide are described and discussed.