Polycations constitute an important group of polymeric materials widely used in the construction of novel drug delivery systems. The positive charge of these compounds renders them highly soluble in aqueous media and allows for strong electrostatic interactions of these polymers with anionic substances, including drugs and substances contained in the body (proteins and nucleic acids, in particular) as well as negatively charged anatomical structures (e.g., cell membranes, glycocalix or mucosa). A wide spectrum of delivery systems containing polycations has been recently developed and studied, including polyelectrolyte complexes, nano- and microparticles, injectable hydrogels, liposomes and micelles. Cationic polymers are also intensively used as penetration enhancers and release triggers. In this chapter the recent developments in the application of polycations for delivery of various therapeutic agents are summarized and discussed. The potential problems related to their potential cytotoxicity and the possible ways of eliminating unwanted side effects are also presented.