Signal transduction is a fundamental cellular process where cells receive and respond to external stimuli through a complex cascade of molecular interactions. This process begins at the cell membrane with receptors binding signaling molecules like hormones, triggering intracellular pathways. Key players include G-protein coupled receptors (GPCRs), which upon activation by agonists, initiate signaling cascades that modulate enzyme activity, gene expression, and ion channels. Second messengers such as calcium ions (Ca²⁺), cAMP, and DAG relay these signals internally, influencing diverse cellular responses like muscle contraction and metabolic regulation. Dysregulation of these pathways is linked to diseases like cancer, highlighting the importance of understanding signal transduction for developing targeted therapies.