The concept of drug delivery systems (DDS) promises the treatments for the refractory diseases such as cancer. Many kinds of multifunctional DDS have been developed, demonstrating enhanced drug accumulation and therapeutic efficiency. However, it is quite difficult to evaluate that the DDS really perform as expected under in vivo conditions. We have recently developed the intravital real-time confocal laser scanning microscopy (IVRTCLSM) characterized by rapid scanning and simultaneous multicolor detection, to visualize the behavior of DDS in living mice. IVRTCLSM revealed the dynamic states of DDS, which could not be observed by conventional in vitro/ex vivo methods, in the bloodstream, tumors, and other organs. IVRTCLSM will provide new facets in the DDS study, and will facilitate the development of DDS.