This paper examines the role of animal- and plant-sourced proteins in achieving a balanced human diet, emphasizing the importance of amino acid (AA) composition and digestibility rather than protein quantity alone. It reviews dietary requirements for essential and nonessential AAs across life stages and compares the nutritional quality of common protein sources, including cereals, legumes, nuts, seeds, meat, dairy, eggs, and emerging alternatives such as insects, algae, and cultured proteins. The discussion highlights complementary protein strategies for vegetarians and vegans, global disparities in protein supply, and future demand projections driven by population growth and evolving dietary preferences. Environmental impacts of protein production—such as nitrogen emissions, greenhouse gas contributions, and land use—are analyzed alongside sustainability considerations. The paper concludes that a balanced mix of plant and animal proteins, supported by sustainable practices and innovative sources, is essential for meeting nutritional needs while minimizing ecological footprints.