After briefly summarizing the outstanding achievements and new tendencies witnessed in recent years in the studies of magnetic fields of solar active regions (ARs), we focus on the current understanding on flux appearance and disappearance. On flux appearance, the moving magnetic features (MMFs) stand still as a mystery in solar physics, although the emergence of magnetic flux in the bipolar form are fairly understood. However, the possibly sympathetic flux emergence of several ARs and the appearance of active longitudes or hot spots are poorly understood. The only confirmed model of flux disappearance is the observed flux cancellation. Detailed analysis of the alignments of transverse magnetic fields and the history of magnetic flux evolution suggest that flux cancellation is more likely to be the magnetic reconnection in the lower solar atmosphere. Magnetic and current helicity provide new diagnosis in understanding flux emergence and disappearance, and constrains the energy process in solar activity.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html