Purpose: In Germany, principals' working environments are data-rich for school improvement purposes in a rather low-stakes test-regime. An effective use of externally as well as internally generated data, also known as data wise leadership, is considered to be a key competence of successful principals. The purpose of this paper is to describe data use practices in German schools, especially focussing on schools facing challenging circumstances. Design/methodology/approach: The paper draws on findings from two projects funded by the German Federal Ministry of Education and Research. A standardized questionnaire study was conducted to measure the usage of available data for school improvement quantitatively. The perceived usefulness, the reflection upon, and the use of information sources by principals were measured. Moreover, semi-structured interviews with school leaders from schools in deprived areas were conducted, transcribed and analyzed by means of qualitative content analysis. Findings: The results showed that internal sources of information were used frequently while (international) student assessment, statewide comparative tests, and school inspections proved to be of relatively little use for the principals' professional practice. Moreover, there was a discrepancy between the perceived usefulness of information sources and the actual use of the data. Originality/value: In contrast to the USA, for example, research on educational leadership and principals' data use - particularly at schools in challenging circumstances - is rather limited in Germany. This paper seeks to take up this desideratum.