Weather radar calibration has remained an active topic of discussion and development since the early days of its introduction to the scientific and operational community. Ever since data has been used quantitatively, calibration has become a very important topic, and an area of great interest at conferences. With the proliferation of dual-polarization radar measurements, there are more demands, and also more exposure of problems that were not known and investigated when reflectivity was the only quantity of interest. Modern digital receivers, advanced microwave switching devices, and stable noise sources have further changed the way we conduct weather radar calibration. Calibration that was performed as infrequently as once a month, or even once a season, now gets done at every scan. In addition the NEXRAD dual polarization upgrade program has emphasized the demand to do calibration in an automated way but with a significant attention to both reflectivity and differential reflectivity. This paper takes a fundamental look at the weather radar calibration process, and demonstrates how modern technology has made many things easier and more accurate, while highlighting the basic challenges . This paper is co-authored by experts who have worked on this topic since the beginning days of weather radar, while at the same time including, current adopters and student practitioners working on the same topic, providing a broad perspective.