Simultaneous Nitrification and Denitrification (SND) is a promising process for biological nitrogen removal. Compared to conventional nitrogen removal processes, SND is cost-effective due to the decreased structural footprint and low oxygen and energy requirements. This critical review summarizes the current knowledge on SND related to fundamentals, mechanisms, and influence factors. The creation of stable aerobic and anoxic conditions within the flocs, as well as the optimization of dissolved oxygen (DO), are the most significant challenges in SND. Innovative reactor configurations coupled with diversified microbial communities have achieved significant carbon and nitrogen reduction from wastewater. In addition, the review also presents the recent advances in SND for removing micropollutants. The micropollutants are exposed to various enzymes due to the microaerobic and diverse redox conditions present in the SND system, which would eventually enhance biotransformation. This review presents SND as a potential biological treatment process for carbon, nitrogen, and micropollutant removal from wastewater.