A holomorphic modular form on an orthogonal group of signature (2,n) is called reflective if its zero divisor is determined by reflections in the orthogonal group. This type of modular forms was introduced by Borcherds, Gritsenko and Nikulin in 1998, and has applications in generalized Kac-Moody algebra, algebraic geometry and number theory. The number of reflective modular forms was conjectured to be finite. The classification of such modular forms has been studied by many mathematicians in the past 20 years. In this talk, I will introduce some new classification results and present some applications.