Context. The use of data warehouses, a specialized class of information systems, by organizations all over the globe, has recently experienced dramatic increase. A Data Warehouse (DW) serves organiza-tions for various important purposes such as reporting uses, strategic decision making purposes, etc. Maintaining the quality of such systems is a difficult task as DWs are much more complex than ordi-nary operational software applications. Therefore, conventional methods of software testing cannot be applied on DW systems. Objectives. The objectives of this thesis study was to investigate the current state of the art in DW testing, to explore various DW testing tools and techniques and the challenges in DW testing and, to identify the improvement opportunities for DW testing process. Methods. This study consists of an exploratory and a confirmatory part. In the exploratory part, a Systematic Literature Review (SLR) followed by Snowball Sampling Technique (SST), a case study at a Swedish government organization and interviews were conducted. For the SLR, a number of article sources were used, including Compendex, Inspec, IEEE Explore, ACM Digital Library, Springer Link, Science Direct, Scopus etc. References in selected studies and citation databases were used for performing backward and forward SST, respectively. 44 primary studies were identified as a result of the SLR and SST. For the case study, interviews with 6 practitioners were conducted. Case study was followed by conducting 9 additional interviews, with practitioners from different organizations in Sweden and from other countries. Exploratory phase was followed by confirmatory phase, where the challenges, identified during the exploratory phase, were validated by conducting 3 more interviews with industry practitioners. Results. In this study we identified various challenges that are faced by the industry practitioners as well as various tools and testing techniques that are used for testing the DW systems. 47 challenges were found and a number of testing tools and techniques were found in the study. Classification of challenges was performed and improvement suggestions were made to address these challenges in order to reduce their impact. Only 8 of the challenges were found to be common for the industry and the literature studies. Conclusions. Most of the identified challenges were related to test data creation and to the need for tools for various purposes of DW testing. The rising trend of DW systems requires a standardized testing approach and tools that can help to save time by automating the testing process. While tools for operational software testing are available commercially as well as from the open source community, there is a lack of such tools for DW testing. It was also found that a number of challenges are also related to the management activities, such as lack of communication and challenges in DW testing budget estimation etc. We also identified a need for a comprehensive framework for testing data warehouse systems and tools that can help to automate the testing tasks. Moreover, it was found that the impact of management factors on the quality of DW systems should be measured.