The electrons on the Fermi surface predominantly determine the physical properties of conductors. Therefore, the information on the Fermi surface is extremely important for understanding the origin of the properties. The conduction electrons undergo cyclotron motions in magnetic fields. The electronic states are then Landau-quantized, which leads to quantum oscillations in the free energyFree energy or thermodynamic grand potential as a function of the magnetic field. The quantum oscillations contain valuable information on the Fermi surface. In this chapter, the Landau quantization of electronic states in magnetic fields and its fundamentals are discussed, which are essential for understanding quantum oscillations. The observation conditions of quantum oscillation are also presented.