Personal Informatics (PI) are information systems that allow people to process activities with the usage of information technology, aiming to produce informational products (data) either for themselves or for others. Technologies that enable PI are becoming increasingly popular, assisting people in collecting personally relevant information about their body and their behaviour. In sports industry nowadays, a great variety of PI wearable tools offer support to athletes and training staff to improve their performance. An example of such tool is the Electronic Performance and Tracking Systems (EPTS), which are a combination of hardware and software that facilitates the collection, storage, analysis and management of professional athletes’ fitness and health data. Although significant and broadly used, EPTS have not yet received much attention from researchers and, thus, understudied. Therefore, the master’s thesis explores the perceptions of professional football athletes and training staff regarding the use of EPTS in their everyday training and work. Furthermore, the master’s thesis research explores the benefits and challenges that professional football athletes and training staff experience when using EPTS in their everyday training and work. The master’s thesis study adopts the interpretive paradigm and qualitative ethnographic approach. The research data was collectedthrough direct observations in the field and semi-structured interviews from Greek professional football athletes and Greek training staff that use wearable EPTS in their everyday training and work and was analysed thematically. A theoretical framework, which is built upon relevant literature from the informatics field and along with the theory of sensemaking, is used to understand, interpret and discuss the research findings. The research outcome of the master’s thesis shows that communication is at the core of EPTS enabling football players and training staff to improve individual and team performance. Organizing of every day starts and ends with EPTS analysis and evaluation and better organized and daily evaluated football methodology appears as key benefit for the club. Coaches and trainers are now more data driven and accurate and analysts and trainers that conduct analysisof the data provided by EPTS are new members of the training staff. Evidence provided by EPTS build trust between staff and players and in the training staff. Visualization tools for presenting insights need to be further improved with the addition of in-field monitors and 3D presentations. Furthermore, it is important for training staff members to have ethical and consistent strategy on how data derived from EPTS are used on how data are communicated.  The research complements previous research on personal informatics and adjusts them to elite team sport context and adds to the theory of sensemaking regarding how users make sense of PI tools that are related with their everyday routines at work. In addition, it offers football training staff members a model for efficient use of EPTS technology into the everyday football practices and a model of sustainable use aiming the overall improvement of team performance.