I present the free energy landscape (FEL) approach to glass transition which provides a unified understanding of glass transition singularities. For quasi-equilibrium systems, thermodynamic quantities can properly be defined with the use of the probability function of each basin of the FEL. I argue that the specific heat of glassy systems consists of three contributions; the glass specific heat, the configurational specific heat and a term related to the temperature dependence of the probability distribution. The FEL frame work is generalized to handle time-dependent phenomena, where time-dependent quantities such as cooling rate dependence can be investigated. I also discuss the temperature modulation spectroscopy on the basis of the FEL theory. Finally, I emphasize the most attractive merit of the FEL theory, namely it can handle thermodynamic and dynamic effects such as the temperature dependence of the crystallization time in a single theoretical frame work.