The market for mobile devices is growing rapidly nowadays. Constant technological improvements provide great opportunities for the creation of mobile applications. For the success of a mobile application or website, one of the main concerns, besides security issues, is usability. Poor usability decreases user productivity and consequently causes loss of users. In order to avoid these problems, usability aspects have to be considered already during the design phase of the application, e.g. by following predefined usability guidelines. Although usability guidelines for web development are already in place since the 1990s, structured and evaluated usability guidelines for mobile applications can rarely be found in scientific literature. Thus, in this paper we introduce a catalogue of usability guidelines for mobile applications and websites, and subsequently demonstrate their usage by applying them in two case studies: the development of a mobile application and a mobile website.