The El Niño Southern Oscillation (ENSO) continues to boast its prominence as Earth’s strongest source of year-to-year climate variability with the appearance of a remarkable El Niño event in the boreal winter of 2015. The 2015/16 El Niño was indeed a strong event with dramatic impact on a global scale. However, it exhibited distinct characteristics from those of past extreme El Niños in modern instrumental record. This challenges our previous understanding of an extreme El Niño which is important for ENSO prediction, monitoring, and future projections. The emergence of the 2015/16 El Niño has diversified the small sample of ENSO events in our short instrumental record. It has facilitated important discussions on our progress in understanding the nature of ENSO and its extremes, how they respond to greenhouse warming, and what the climate science community should do next in their quest to fully grasp the complexity of ENSO behavior. These are covered in this review paper which establishes the 2015/16 El Niño as the first extreme El Niño of the 21st Century.