The long standing questions of the origin of the tiny neutrino mass and the observed flavor mixing have no answer within the Standard Model (SM). To explain such behaviors we need to extend the SM. A plethora or models or scenarios have been proposed to investigate the origin of the neutrino mass. In this talk we will discuss about simple extensions of the SM where SM-singlet heavy Majorana Neutrinos are introduced to generate the light neutrino mass from the seesaw mechanism. Also we will discuss the testability of such scenarios at the energy frontier such as Large Hadron Collider (LHC) and electron-positron collider.