Hydrates are commonly observed in pharmaceutical development.For example,about one third of pharmaceutical compounds exist as hydrates,and several antibiotic agents can only be crystallized as hydrates.There are several types of hydrates in which associations between water and drug molecules may vary considerably.Furthermore,the conversion between hydrates and non-hydrates may exhibit different kinetics and pathways,which can significantly impact crystallization,stability and scale-up process.Understanding hydrate formation and hydrate stability can be critical in solid form selection and process control in manufacturing of drug substances and drug products.In this presentation,classification of hydrates,conversion pathways and kinetics of hydration and dehydration and examples of pharmaceutical hydrates investigated using various solid-state techniques will be discussed.