This brief describes studies conducted by the authors on mid-size drugs utilizing peptides and peptidomimetics, and on the development of anti-HIV agents. Peptides are important biological molecules and have various physiological actions. Peptide-based drug discovery may help bring about the development of useful medicines that are highly safe and show potent pharmacological effects in small doses. Recently, it has been shown that there is an important drug-like space in the mid-sized region between low- and high-molecular-weight compounds. Thus, mid-size drugs such as peptide compounds are being focused on. To date, several peptidomimetics that mimic primary, secondary, and tertiary structures of peptides have been developed to maintain and improve biological activities and actions of peptides. In this book, the features and advantages of mid-size drugs are described in detail. In addition, the merits of utilizing peptidomimetics in the development of mid-size drugs are referred to. Understanding such peptide-derived mid-size drugs will lead to a comprehensive expansion of medicinal chemistry.