Disclosed is a thin film transistor substrate which facilitates to realize a bottom gate structure where a gate electrode is disposed below an active layer, and to increase an area for a storage capacitor, and a display device including the same, wherein the thin film transistor substrate may include a light shielding layer, a buffer layer for covering the light shielding layer, and a driving transistor prepared on the buffer layer while being overlapped with the light shielding layer, and provided to supply a driving current to an organic light emitting device.