We explore the education difference in knee pain among middle-aged and older individuals. Knee pain differs greatly by education and is a key reason why disability insurance differs so much. We first show that knee pain is not strongly related to arthritis in the knee. Less than one-quarter of the difference in pain is a result of knee arthritis. That said, the knee pain reports appear ‘real’; they correlate with physical functioning and subsequent receipt of medical care. We test several theories for the differential rate of knee pain by education. We show that physical demands on the job and obesity each explain about one-third of the gradient in knee pain. There is little to no impact of a more positive mental outlook or of differential sensitivity to stressful situations in explaining knee pain. As people continue to become more obese and jobs grow in areas with physical demands such as home health care and personal assistance, our results suggest that knee pain and associated disability may continue to increase.