Abstract During object-oriented system development, programming should be preceded by analysis and design to assure that the system fulfils the demands of the customer and simplify during the development phase and documentation. When modeling the analysis and design phases, several different notations may be used. One of these is the UML (Unified Modeling Language) which this thesis will cover. The aim is to compare the use of the UML i practice versus what is said in the literature. The investigation is built upon interviews at different companies to receive their reflections about the UML. Questions at issue are why and when the selected companies use the UML and what diagrams they use. We also investigate whether they strictly follow the UML notation or complement it with another kind of notation. Moreover two companies not using the UML was interviewed to find out why they have chosen not to. The thesis starts with an introduction to object-oriented system development with analysis and design followed by the history of the UML and its most common diagrams according to Larman [1]. These diagrams are use case, conceptual model, system-sequence diagram, contract, interaction diagram, class diagram and state diagram. Each diagram is explained with text and graphics. The most important results are that the UML is considered being adequate to the system developers. They use a number of the most common diagrams. Furthermore the CASE-tools showed not to meet the demands of the developers. We believe that inadequate tools hinders the future diffusion of the UML on the market. Improved tools for modeling and documentation are desired for all of the interviewed companies.