The purpose of this thesis is to compile theoretical information needed in the development of inventory and purchase module to an enterprise resource planning system, and to study what needs Nestix Ltd's clients and some other companies have for this software. The theory and the needs will be combined to formulate a rough definition of needs for the software, on the basis of which Nestix Ltd will then realize the new modules for its enterprise resource planning system. In the study, it was discovered that different companies have very similar needs for their purchase and inventory controls. The purchase and inventory processes have minor differences between different companies. The same traditional basic tools of logistics are used in almost every company. The most tempting field of development in the future would be the internet base of the softwares, which allows the use of the software from all over the world without distinct software installations. After the study, the integrated purchase and inventory modules were realized and added to the Nestix software family. Since the study, the software has been developed further, and it has been sold to several European countries as a part of Nestix enterprise resource planning system.