The book reveals the overall importance of a customer relationship management system, especially for small and medium-sized enterprises. In addition to the topic of CRM, it discusses the increasing importance and possibilities of open source software. The main research question explores whether open source customer relationship management systems can fulfill the requirements of CRM software. To answer this question, the analysis utilizes available literature on CRM, the specific requirements of small and medium-sized enterprises, and open source software. By identifying what a CRM must fulfill to be classified as a customer relationship management system according to the literature, various requirements are outlined. In the next step, the three most popular open source CRM software systems are examined.