In this book, we will review the most recent studies about the macroscopic and microscopic structure of the synovium, its function in both normal and pathological conditions. We will describe the theories that relate to synovitis as a risk factor for osteoarthritis of the knee, as well as possible treatments focused on the synovium to prevent the progression of osteoarthrosis of the knee, and finally, we will describe the recent studies on the management of the synovial membrane during total knee arthroplasty.