Essential amino acids are a group of nine amino acids that the human body cannot synthesize on its own and thus, must obtain from dietary sources. This concept emerged in the early 20th century when scientists discovered the importance of amino acids for plant and human nutrition. These indispensable amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. They serve numerous functions in the body such as supporting protein synthesis, hormone balance, and immune function. Foods that contain all nine essential amino acids, such as meat, eggs, and soy, are termed complete proteins. Throughout history, understanding the role of these amino acids has been crucial for advancements in nutrition science, especially in designing a well-balanced diet.