The Quantum Cascade Laser is a new opto-electronic semiconductor laser emitting in the infrared and terahertz regions of the electromagnetic spectrum. The lasing is achieved by tailoring the widths of thin semiconductor layers to achieve population inversion between specific electronic states in the coupled quantum wells. The many design parameters together with the complex transport properties, where both scattering and coherent evolution are important, have made these devices interesting for theoretical studies. In this thesis, we study different aspects of the gain profile. After a short intoduction of the laser device and calculation method, we present two different studies regarding the evolution of the gain/absorption spectra when varying the applied bias and the temperature.