Peptide amphiphiles (PAs) are novel engineered biomaterials able to self-assemble into supramolecular systems that have shown significant promise in drug delivery across the cell membane and across challenging biological barriers showing promise in the field of brain diseases, regenerative medicine and cancer. PAs are amino-acid block co-polymers, with a peptide backbone composed usually of 8-30 amino acids, a hydrophilic block formed by polar amino acids, a hydrophobic block which usually entails either non-polar or aromatic amino acids and alkyl, acyl or aryl lipidic tails and in some cases a spacer or a conjugated targeting moiety. Finely tuning the balance between the hydrophilic and hydrophobic blocks results in a range of supramolecular structures that are usually stabilised by hydrophobic, electrostatic, β-sheet hydrogen bonds and π-π stacking interactions. In an aqueous environment, the final size, shape and interfacial curvature of the PA is a result of the complex interplay of all these interactions. Lanreotide is the first PA to be licensed for the treatment of acromegaly and neuroendocrine tumours as a hydrogel administered subcutaneously, while a number of other PAs are undergoing preclinical development. This review discusses PAs architecture fundamentals that govern their self-assembly into supramolecular systems for applications in drug delivery.