In order to make informed decisions about the timing of harvests of grass crops, it is helpful to be aware of the changes that are taking place in the crops as they grow. Most people growing grass commercially or experimentally have a general awareness of the more obvious changes. However, greater awareness and understanding should lead to greater precision and greater success in the relevant enterprises. The present review considers some of the more important changes that are of interest from an agricultural point of view and for which sufficient information is available in the scientific literature. It is intended for relevant students, lecturers, research workers, advisers and producers.