Cross-Project-Defect Prediction as a sub-topic of defect prediction in general has become a popular topic in research. In this article, we present a systematic mapping study with the focus on CPDP, for which we found 50 publications. We summarize the approaches presented by each publication and discuss the case study setups and results. We discovered a great amount of heterogeneity in the way case studies are conducted, because of differences in the data sets, classifiers, performance metrics, and baseline comparisons used. Due to this, we could not compare the results of our review on a qualitative basis, i.e., determine which approaches perform best for CPDP.