Between a construction idea and its implementation creativity, engeneering skills, time, money, devices, human ressources, material, etc. are required. Some projects cannot be handled by all contractors because of their immense volume. The only possibility for some contractors to be part of the competition is establishing a bidding consortium. Thereby some construction companies are combined to submit a joint. In case of winning the competition, the companies sign a contract to operate as a joint venture and to be liable solidary. At the beginning of this master thesis, a chapter presents general information about joint ventures and contains details on their characteristics and their forms of their occurance as well as the definition of joint ventures. The advantages and disadvantages of joint ventures for contractors and employers are also listed and the characteristics of bidding markets in the construction industry are described. The following chapters of this master thesis deal with the phases of a project and their specific regulations concerning to joint ventures. The main chapter of this master thesis discusses the cartel law and its influence on joint ventures. This topic is already briefly mentioned in the first part of this master thesis, when the process planing is discussed. Breach of the cartel law generally means a created restraint of competition caused by all agreements between undertakings, decisions by associations of undertakings and concerted practices. According to this, joint ventures, being powerful contractors, are suspected to break the cartel law because of their big market influence. How this border line of allowed market influence is defined will be answered in this thesis, as well as: -Do joint ventures always constrain the competition because of the induced reduction of the amount of bidders?-. Additionally, a comparison of the national and the European law and their commonalities as well as their differences forms an important part of this master thesis. Furthermore, the wording of the cartel law as well as the fundamental components to regulate the competition and their relationship will be presented. The competitive evaluation of the cartel law referring to the respective joint venture that has to be completed by every joint venture before submitting an offer is described in detail. In order to reach a clearer overview about this complex topic, the author created a flow diagramm. This master thesis concludes with an analysis of interviews with experts from the construction industry. Three contractors and three employers where asked about their opinion on joint ventures and the influence of the cartel law as it applies to them.