Osteoarthritis is a major source of disability in developed countries. As populations age, we can expect knee OA to become a serious public health problem. Preventative strategies to minimise the risk of both the development and progression of knee OA are therefore of paramount importance not only with respect to quality of life issues but the burdening costs of managing and treating this common disorder in the next few decades. The focus of preventative strategies should be on the modifiable risk factors for both incident and progressive disease. The aetiology of OA of the knee is complex and multifactorial. This review focuses on the modifiable environmental risk factors for knee osteoarthritis; namely occupation, physical activity, quadriceps strength, joint injury, obesity, diet, sex hormones, and bone density. Their contribution is well understood and the impact of altering their influence on knee OA outcome is also now being evaluated. Since they are modifiable, this has important implications for public health recommendations and treatment by health professionals.