Quantum cascade detectors are intersubband devices that offer bright prospects for photodetection of mid-Infrared radiations because of high detection speed, reliable room temperature operation and excellent Johnson noise limited detectivity. This is achieved thanks to the photovoltaic operating principle allowed by their design, comprising an internal potential gradient in the form of a quantum cascade, hence the name of the devices. In this work, the experimental setup to measure the spectral responsivity of such photodetectors is described, together with all aspects of their physics. Moreover the improvement of device performance and the investigation of an optimal architecture in terms of detectivity are discussed, relying on the key concept of quantum engineering of the bandstructure.