We all know that hay is important to rabbits and herbivorous companion rodents, but understanding why it is so important makes all the difference. In this session, we will review the evolution of grasses and spread of grasslands, and discuss how that influenced the evolution of rabbits and herbivorous rodents. We will also review how hay, or more specifically the lack thereof, leads to the development of dental disease in rabbits, guinea pigs, and chinchillas.