Electromigration (EM) problem in Al interconnects is one of the most persistent and important challenges in the microelectronic industry. From 1990s, with the scaling down and new processes introduced, ultra-deep submicron (feature size≤0.18 μm) Al interconnects involve more complicated EM issues. In this paper, we summarized the basic formulae of the EM phenomena and got the important flux divergence equations. Based on the equations, the research methods for EM problems were reviewed. The main issues and solutions during the half century of investigation for Al EM challenges were reviewed as well. Finally, the essentials and challenges of current ultra-deep submicron Al EM reliability are analyzed, and some prospective solutions were also proposed.