Implantation of total knee and hip endoprosthesis has become a routine procedure carried out in hospitals around the world with the purpose of restoring patient's everyday functions and quality of life. The end result depends on the ability of the surgeon carrying out the procedure and on the design parameters and build quality of the prosthesis itself. Each endoprosthesis is individually adjusted to the patient and his or hers physical characteristics. By observing the human body, researchers try to gain insight about the characteristics which need to be considered while designing and producing the endoprosthesis, in attempt to imitate the original state as naturally as possible. In this paper, the anatomy of knees and hips is explained so the reader acquires knowledge necessary to understand the terms discussed in the later parts of the paper. Next the characteristics of the endoprosthesis itself are described and also different types of endoprosthesis are classified based on the implantation methods and function. Kinematic relations in the endoprosthesis itself and the relations inbetween the other body parts and the endoprosthesis are analysed. Forces and loads which act on the endoprosthesis are also analysed by studying the forces that act on the natural joints. In the end, possible finite element method solutions are considered by using software Ansys.