膝，俗称膝盖，为位于大小腿之间的连接部位。膝的主要内部组成结构为半月板以及四条韧带。

半月板为膝内部股骨下端和胫骨上端之间的接连处所垫的一块新月形的纤维软骨组织，作用是缓冲膝关节的震动，以及避免两块骨头的直接摩擦。

另外，它由四条主要的韧带支撑着。其中有两条在膝的两侧，称为中侧突韧带和外侧突韧带，主要功能是防止膝部轴离位；另外两条韧带则分别分布在膝前后方，称为前十字韧带和后十字韧带，作用是防止膝部前后移位。

The knee joint joins the thigh with the leg and consists of two articulations: one between the femur and tibia, and one between the femur and patella. It is the largest joint in the human body. The knee is a mobile trocho-ginglymus, which permits flexion and extension as well as slight internal and external rotation. The knee joint is vulnerable to both acute injury and the development of osteoarthritis.