Using natural excipients in pharmaceutical formulations has gained significant attention due to their biocompatibility, safety, and sustainability. Among these, proteins and peptides emerge as promising candidates owing to their multifunctional properties, including their ability to stabilize drug molecules, enhance bioavailability, and act as carriers for targeted drug delivery. This chapter delves into the structural and functional attributes of proteins and peptides that make them suitable as pharmaceutical excipients. This chapter also highlights various quality control aspects of proteins/peptides using orthogonal analytical techniques. It explores various natural sources, such as albumin, gelatin, and casein, and their roles in solid and liquid formulations. Furthermore, this chapter highlights the advantages and challenges of using these biopolymers, including stability, immunogenicity, and scalability issues. The discussion aims to provide insights into the future potential of proteins and peptides in advancing the development of more effective, patient-friendly pharmaceutical products. This chapter also discusses regulatory considerations and modifies the naturally occurring protein/peptide to enhance biocompatibility.