Insulin is a polypeptide hormone produced by pancreatic β-cells, regulating glucose metabolism, lipid storage, and protein synthesis. Insulin resistance (IR), characterized by reduced insulin signaling, leads to hyperinsulinemia and is linked to metabolic disorders like type 2 diabetes and obesity. IR also increases cancer risk and poor outcomes. This review discusses the mechanisms, causes, and types of IR, focusing on cellular changes. Insulin signaling involves the insulin receptor (INSR), which activates pathways like PI3K/AKT and MAPK. These pathways regulate glucose uptake, glycogen synthesis, and protein metabolism. IR disrupts these processes, leading to metabolic dysfunction. IR is caused by genetic mutations, environmental factors, or autoimmune responses. Severe IR syndromes, such as Donohue syndrome and Rabson-Mendenhall syndrome, are rare and involve impaired insulin signaling. IR also affects other organs, including the liver, muscles, adipose tissue, and brain, contributing to metabolic and cardiovascular diseases. IR is associated with increased risk of cancer, cognitive decline, and other conditions. Mechanisms of IR include defects in insulin signaling pathways, ectopic lipid accumulation, and the hexosamine biosynthesis pathway. Understanding these mechanisms is crucial for developing strategies to combat IR.Insulin is a polypeptide hormone produced by pancreatic β-cells, regulating glucose metabolism, lipid storage, and protein synthesis. Insulin resistance (IR), characterized by reduced insulin signaling, leads to hyperinsulinemia and is linked to metabolic disorders like type 2 diabetes and obesity. IR also increases cancer risk and poor outcomes. This review discusses the mechanisms, causes, and types of IR, focusing on cellular changes. Insulin signaling involves the insulin receptor (INSR), which activates pathways like PI3K/AKT and MAPK. These pathways regulate glucose uptake, glycogen synthesis, and protein metabolism. IR disrupts these processes, leading to metabolic dysfunction. IR is caused by genetic mutations, environmental factors, or autoimmune responses. Severe IR syndromes, such as Donohue syndrome and Rabson-Mendenhall syndrome, are rare and involve impaired insulin signaling. IR also affects other organs, including the liver, muscles, adipose tissue, and brain, contributing to metabolic and cardiovascular diseases. IR is associated with increased risk of cancer, cognitive decline, and other conditions. Mechanisms of IR include defects in insulin signaling pathways, ectopic lipid accumulation, and the hexosamine biosynthesis pathway. Understanding these mechanisms is crucial for developing strategies to combat IR.