Technical Abstract: Ruminants grazing biodiverse pasture face many choices, including when and where to graze and how much herbage to consume. Scientific research has led to considerable knowledge about some of these choices (e.g. herbage DMI), but other aspects of the complex decision-making process of a grazing ruminant remain less clear. These choices not only affect the nutritional status of the animal, but also affect the sward through selective defoliation. Understanding and managing animal preferences and thereby altering dietary selection, can result in greater primary (animal) and secondary (plant) productivity of this complex dynamic system. This paper reviews the current state of knowledge related to diet selection of grazing domestic ruminants with particular reference to temperate grazing environments, including how well we understand each part of the complex decision-making process a grazing ruminant faces, the links with primary and secondary productivity, and developments in methodologies. Finally, we identify key areas where knowledge is lacking and further research is urgently required.