Contribution barriers are properties of Free/Libre and Open Source Software (FLOSS) projects that may prevent newcomers from contributing. Contribution barriers can be seen as forces that oppose the motivations of newcomers. While there is extensive research on the motivation of FLOSS developers, little is known about contribution barriers. However, a steady influx of new developers is connected to the success of a FLOSS project. The first part of this thesis adds two surveys to the existing research that target contribution barriers and motivations of newcomers. The first exploratory survey provides the indications to formulate research hypotheses for the second main survey with 117 responses from newcomers in the two FLOSS projects Mozilla and GNOME. The results lead to an assessment of the importance of the identified contribution barriers and to a new model of the joining process that allows the identification of subgroups of newcomers affected by specific contribution barriers. The second part of the thesis uses the pattern concept to externalize knowledge about techniques lowering contribution barriers. This includes a complete categorization of the existing work on FLOSS patterns and the first empirical evaluation of these FLOSS patterns and their relationships. The thesis contains six FLOSS patterns that lower specific important contribution barriers identified in the surveys. Wikis are web-based systems that allow its users to modify the wiki's contents. They found on wiki principles with which they minimize contribution barriers. The last part of the thesis explores whether a wiki, whose content is usually natural text, can also be used for software development. Such a Wiki Development Environment (WikiDE) must fulfill the requirements of both an Integrated Development Environment (IDE) and a wiki. The simultaneous compliance of both sets of requirements imposes special challenges. The thesis describes an adapted contribution process supported by an architecture concept that solves these challenges. Two components of a WikiDE are discussed in detail. Each of them helps to lower a contribution barrier. A Proof of Concept (PoC) realization demonstrates the feasibility of the concept.