Tao Wang is a professor at the School of Astronomy and Space Science, Nanjing University. He received his Ph.D. degree from Nanjing University in 2012. He was a postdoctoral fellow at CEA-Saclay, France, from 2013 to 2016, and at Tokyo University, Japan, from 2017 to 2020. He joined Nanjing University in 2021.His research focuses on the formation and evolution of massive galaxies and galaxy clusters over cosmic time, using multi-wavelength observations from the world's most powerful optical, submillimeter, and radio telescopes.In the Lambda-CDM Universe, massive galaxies populate massive dark matter halos. As a result, the cosmic evolution of the number density of massive galaxies  provides critical constraints not only on theories of galaxy formation but also on cosmology.  While the number density and physical properties of massive galaxies in the nearby Universe are relatively well understood, their number and properties in the early Universe have been actively debated. Earlier studies based on optical/ultraviolet observations generally reveal a deficit of massive galaxies at high redshifts. However, recent JWST studies appear to support a much larger population of massive galaxies in the early Universe that is even beyond expectations of  Lambda-CDM cosmology. In this talk, I will discuss the underlying physics causing these observational discrepancies, and  show that with the help of mid-infrared data from JWST/MIRI, we can finally obtain definitive constraints on the true number density of massive galaxies at high redshifts. With this MIRI-selected complete sample of high-redshift massive galaxies, I will further discuss how they open up new opportunities to solving some longstanding problems in galaxy formation, including the cosmic star formation rate density,  the star-forming main sequence, and the fraction of quiescent galaxies at high redshifts.