Nanoparticles for a selective, two stage delivery to tumors have been developed. The nanoparticles are initially sized so that they preferentially accumulate in the tumor tissue as a result of leakage through the defective vascular in the solid tumors. Once in the tumor tissue, the nanoparticles are cleaved hydrolytically and/or by enzymatic cleavage over time to release smaller nanoparticles carrying therapeutic, prophylactic or diagnostic agents into the necrotic interior of the tumors. This provides a simple, elegant and highly effective means of delivery drug selectively not just to tumors generally, but, more importantly, into the poorly vascularized necrotic interiors which drugs are normally unable to penetrate. The nanoparticles have a number of advantages: less toxicity due to selective accumulation only in the tumors; access into the poorly vascularized necrotic interiors of the tumor; and sustained release over a period of time within the tumor.