Currently, most of the reported small molecular anti-cancer drugs have some inherent drawbacks, such as poor water solubility, high drug dosage, short drug half-life in vivo and so on. After taken by oral administration or intravenous injection, these anti-cancer drugs can permeate into cells only by the approach of free diffusion. In general, they can not only kill cancer cells but also normal ones due to lacking of selectivity, and even cause secondary injury to patients. As a result, the clinical applications of these anti-cancer drugs are limited to a large extent. The anti-cancer drug delivery systems (such as micelles, nanogels and nanoparticles, etc.) constructed from the suitable carrier materials may not only prolong the circulation in blood compartments and reduce side effects to normal tissues, but also enhance the water solubility as well as improve the bioavailability of the small molecular anti-cancer drugs. Thus, more and more attentions are paid to them by researchers and pharmacy companies. Up to now, anti-cancer drug delivery systems have advanced over 60 years, and the evolution can be divided into three stages approximately. In this review, three different development stages and the recent progress of anti-cancer drug delivery systems are summarized. Besides, the future development of anti-cancer drug delivery systems is also prospected.

Contents
1 Introduction
2 Anti-cancer drug delivery system
2.1 The first generation of drug delivery system
2.2 The second generation of drug delivery system
2.3 The third generation of drug delivery system
3 Outlook