Rehabilitation Engineering’s work consists of developing products associated with physical rehabilitation, mainly prostheses for upper and lower extremity amputees. Developing such products implies mechanical design and the suitable selection of materials and manufacturing process to replace some of the functions which a patient may have lost. The work presented here was aimed at establishing an optimal solution for substituting the needs of a patient having had the forearm amputated at the proximal level of the elbow. This article describes the whole process followed, from understanding the medical language to developing a functional prototype able to replace the arm’s two functions (i.e. the hand and forearm’s pronation and supination movements).