Drug delivery systems is an umbrella term describing different types of carrier-drug and carrier-protein conjugates, liposomes, polyplexes, micelles, and hybrid types. Such systems were designed with the aim to enhance delivery and to improve the selectivity and pharmacological properties of both conventional drugs and newly developed agents. In this review the current state-of-the-art in the design of drug delivery systems is described, with particular emphasis on agents developed to deliver antitumor agents. The advantages, limitations, and disadvantages of the particular types of drug delivery systems as well as the theoretical and practical basis of their action are also highlighted here. In addition, results of preclinical and clinical studies on the most developed examples of drug delivery systems to date are summarized.