Abstract. In 1998, Super-Kamiokande came up with the solution of long-standing atmospheric neutrino anomaly in terms of neutrino flavour oscillation providing an exclusive evidence for physics beyond the Standard Model. This year we are celebrating the 20th anniversary of discovery of atmospheric neutrino oscillation. In the first part of my talk, I will discuss the physics behind the atmospheric neutrino oscillation. Then, I will discuss the physics reach of currently running and upcoming atmospheric neutrino experiments. In the second part of my talk, I will present the status and prospects of the India-based Neutrino Observatory (INO) facility, a flagship mega-science project of our country to study atmospheric neutrinos. Towards the end of my talk, I will show some selected results from our recent studies on the physics capabilities of the INO experiment.