Therapeutic nanoparticles serve several advantages over systemic administration of drugs, such as minimizing side effects, overcoming dose-limitations, and averting drug resistance. In some cancer types, targeting can be established by size, due to the 'enhanced permeability and retention' (EPR) effect. In rapidly growing tumors, newly formed blood vessels are leaky compared to healthy tissue, which allows particles with diameters between 20 and 200 nm to penetrate the vessel walls through inter-endothelial gaps and accumulate mainly in the tumor region. Recently, other mechanisms for this accumulation are being discussed as well, where particles are transported into the tumor via transcytosis, vesicular organelles, and/or immune cell migration. These mechanisms are summarized under the term 'active transport and retention' (ATR). Both targeting effects, EPR and ATR rely on the permeability of the specific tumor type and the circulation times determined by the particle size and its surface properties. A drug delivery system requires an efficient mechanism for releasing the drug molecule.