Inositols are found ubiquitously in the biological kingdom, and their metabolites play important roles in stress responses, membrane biosynthesis, growth regulation, and many other processes. In this chapter, we describe the role of inositol and its derivative metabolites in abiotic and biotic stress responses. Inositol and its metabolites function as both osmolyte and secondary messengers under these stresses. The accumulation of osmolytes during osmotic stress is a ubiquitous biochemical mechanism found in all organisms from bacteria, fungi, and algae to vascular plants and animals. The accumulated osmolytes include glycerol, myo-inositol, betain, taurine, proline, trehalose, and raffinose. Plants accumulate many kinds of inositol-derivative metabolites during abiotic stresses, such as drought, low temperature, and highsalinity stresses; in contrast most animals accumulate only myo-inositol. In animal systems, it has been well documented that a variety of phosphoinositides and inositol phosphates function as secondary messengers in various signaling processes. Phosphoinositide-specific Phospholipase C (PIPLC) digests phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) to generate two secondary messengers, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and diacylglycerol (DG). (Ins(1,4,5)P3) induces the release of Ca2+ into cytoplasm, which in turn causes various cellular responses. In plants, similar systems function in response to abiotic stress, such as drought, cold, and high-salinity stresses.