The anti-cancer agents or tumor specific targeted agents are conjugated to biodegradable and biocompatible polymers via an ester bond or other chemical bonds and present as nanoparticles. After the nanoparticles bind to the surface receptors (preferably tumor specific) of the tumor cells, they enter into the cells through endocytosis. In the tumor cells the linkage is hydrolyzed by specific enzymes or/and low or high pH in the tumor cells, at which time the nanoparticles are deconjugated and release the free drugs, and antitumor activities begin. The self-assembled and biodegradable nanoparticles can be used as diagnostics or therapeutics depending on what agents or drugs are conjugated into polymers and ligands. Therefore, these nanoparticles allow drug delivery as well as imaging of tumor tissues in future cancer research. For details, see Sinha et al. in this issue.