Hydration is the body’s ability to absorb water and to maintain the correct balance of fluid and electrolytes and is essential to human health. Dehydration can adversely affect metabolism, thermoregulation, digestion, and neurological, kidney, and heart function. Aging as well as disease and medications affect water and electrolyte levels in the body and can lead to dehydration in older adults. In this review, we discuss factors contributing to dehydration in older adults, how hydration is measured, and strategies to improve hydration status. We close with a summary of the different areas of focus related to hydration research.