Moisture content determination is one of the most important and common assessments made on seeds. It contributes to the estimation of the value of a seed lot in commercial settings and the physiological status of seeds in research and conservation. The most common method for determining seed moisture content is by measuring the change in weight of a sample after drying at a constant temperature for a set period of time. This, however, renders the seeds unusable for other purposes, such as viability testing. Here, we review various methods for assessing the moisture status of seeds, including non-destructive methods. Controlling the amount of water in seeds is just as important as being able to measure the amount of water in seeds. Perhaps most notably, orthodox seeds are normally dried to low moisture content to maintain viability and vigour for longer, with the seeds either stored in breathable containers in a dry environment or sealed in air-tight containers such that they cannot reabsorb water. However, and especially in seed research, we are sometimes interested in having seeds at an intermediate or high moisture content. Hence, in this review we also discuss various ways to adjust the moisture content of seeds.