Pharmaceutical stability refers to a substance's capacity to maintain the predetermined levels of identity, potency, and purity over the period of its shelf life. It measures how well a product maintains its original attributes and traits during storage and use, if certain parameters are met. A pharmacological material may be subjected to situations that could significantly affect its chemical and physical integrity during synthesis, processing, and storage. A medicinal substance's chemical stability is extremely important. A marketable medicine must maintain its stability under a range of circumstances, such as extremes in temperature and relative humidity. Pharmaceuticals' chemical instability affects their therapeutic efficacy and toxicological effects. Determining the proper controls to implement during product production, processing, and storage can be made easier with knowledge of the factors that cause the parent chemical to degrade. This chapter talks about various factors affecting chemical stability of drugs and parameters to meet the chemical stability requirement of the drug or formulation.