Polymer crystal hydrates (PCHs) are crystalline solids that form between a polymer and water. To date, only four distinct PCHs have been discovered – one of polyoxa- cyclobutane (POCB) and water, and three different polymorphs of polyethyleneimine (PEI) and water. These PCHs were first reported decades ago andhave fascinating structures and peculiar properties that make them potentially useful for a wide range of applications including refrigeration, proton conduction membranes, and desalination. This perspective revisits what is known about these compounds, categorizes their sim- ilarities and differences with other known compounds, and offers a perspective into future efforts to discover new PCHs to address technological needs for society.