Aluminum (Al) is the most abundant metal element in earth crust, together with low cost and high safety. Al-ion batteries (AIBs) have been regarded as potential alternatives to lithium-ion batteries (LIBs) in large scale applications and attracted much attention in current days. In this review, recent developments of AIBs including the electrolyte exploration, electrode design and Al protection are summarized. Both aqueous and non-aqueous electrolytes exhibit important benefits for AIBs, especially ionic liquid electrolytes with high stripping/plating efficiency, which are preferentially discussed here. Furthermore, we highlight the design principles and electrochemical mechanism for carbons, metal compounds as well as new-type positive electrode materials for high-performance AIBs. Besides, we focus on the negative electrode protection with suitable coating layers to reduce dendrite formation and improve electrochemical activation of Al negative electrodes. The accessible characterization techniques that promote the development of AIBs are discussed. Finally, prospects and outlooks of AIBs towards theoretical investigations and practical applications are provided.