This book examines the endoplasmic reticulum and its involvement in the physiopathological processes in plants and humans. It emphasizes how aging promotes morphological, (ultra)structural, physiological, and functional changes in the endoplasmic reticulum. The modulation of endoplasmic reticulum stress and an enhancement of the intensity of the unfolded protein response are potential strategies to prevent/treat different diseases and promote a global healthy state. A thorough understanding of these mechanisms and their role in physiology and pathophysiology conditions is fundamental in the development of new endoplasmic reticulum-targeted therapies.