This review focuses on the application of two classes of peptides, i.e., self-assembling peptides (SAPs) and cell-targeting peptides (CTPs), in the development of nanocarrier delivery systems. Self-assembling peptides are emerging in a wide range of biomedical and bioengineering applications and fall into several classes, including peptide amphiphilies, bolaamphiphile peptides, cyclic peptides, and ionic complementary peptides, which can be found naturally or synthesized. The advantage of synthesizing peptides is that their self-assembling properties can be exploited to form desirable structures for various applications. Another, unique property of self-assembling peptides, is stimuli-responsibility in different environments including various pHs, temperatures, ionic strengths, etc. These characteristics make peptides applicable in a wide range of biomaterials in drug discovery. This study reviews the design principles of well-known self-assembling peptides, as well as their physical/chemical properties. In addition, it discusses the therapeutic cancer-targeting peptides and current combinatorial peptide library methods used to identify targeting peptides. Cancer-targeting peptides can target either tumor cell surfaces or tumor vasculature. The RGD peptide is one of the first tumor-targeting peptides that can bind to self-assembling peptides or any other nanocarrier to improve the therapeutic efficiency of targeting drug delivery systems.