The theme of this undergraduate thesis is to develop a web application for business management of a fitness center. Web application was developed by using up-to-date technologies for web applications development. In the first part of this thesis, a detailed analysis of all technologies used in the development of this web application was conducted. There is given a description of HTML and CSS technologies which are used for design and building user interface of most of today’s applications. Subsequently, there is given a detailed overview of JavaScript programming language which is used on the client as well as the server side. Next is an introduction of Node.js platform which acts as a web server and allows the application to connect to a database. There is also a description of relational PostgreSLQ database in which are stored all data displayed on the user interface. The second part of undergraduate thesis is practical part and includes the development of the application itself using the previously described technologies. The objective of this application is to facilitate fitness service users and trainers everyday life of through the implementation of functionalities such as: joining a fitness center, purchasing training plans, booking training appointments and reviewing your own profile and training schedule During the development of fitness center web application, it is necessary to successfully master all the technologies that were used in its development which was an additional challenge. Development of this web application as part of undergraduate thesis has helped in understanding the architecture and the way of how web application work. Also, it provided a better understanding of all these technologies and their role in the implementation of the application itself.