Inositols are natural compounds present in animal and plant cells and play a key role in glucose metabolism, acting as second messengers of insulin. They have been shown to be able to counteract the downstream consequences of insulin resistance, exerting beneficial effects on metabolic diseases, infertility and polycystic ovary syndrome (PCOS). We summarize the mechanisms of action of inositol compounds, focusing on the most important functions of myoinositol and D-chiro-inositol in the treatment of metabolic syndrome, hyperlidaemia, insulin resistance and PCOS.