As a non-destructive local gas breakdown phenomenon within a surface or bulk cavity surrounded by insulation, partial discharge (PD) contains important information which can be used to evaluate and diagnose electrical insulation systems. In this paper, distinctive characteristics of PD for DC versus AC are reviewed. A summary of experimental studies on DC PD in different insulation coordinations is presented, including PD in gaseous insulating media, PD in solid insulating media and PD in liquid insulating media. Conclusions and some thoughts based on existing studies are provided, and challenges and suggestions for future studies are given. This paper can serve as a guide reference for readers to learn the background and state-of-the-art of DC PD studies. More importantly, it is hopeful that this study can inspire novel ideas for further advancing fundamental research of DC PD mechanisms and PD mitigation methods.