The topic of this master thesis is ETL systems. The main goal of this thesis was to explain what ETL systems are, what they are for, what are its constituent processes and where we can find them in use. At the beginning of this paper, we deal with the data destination, which we extract and transform using ETL, that is, the data warehouse. The paper describes the main characteristics of the warehouse, enumerates and defines its main components, and a chapter is dedicated to the architecture of the data warehouse. After defining the destination and purpose of the stored data, we described the process by which the data comes to the storage: extracting data from various sources, processing the data and finally storing the data in the warehouse where it becomes valuable information. Using the example of designing and building an ETL system for the MeteroPollen database, we explained how to create your own customized ETL system from scratch. As part of this illustrative example, parts of the source code written in the Python programming language are attached. A separate chapter is dedicated to the types of ETL tools, where several examples of commercial solutions of each type are listed. At the end of this paper, we mentioned some of the advantages of using commercial tools and pointed out when it is better to create your own solution.