Despite the simple detection principle, a gravitational wave interferometer is an extremely complex instrument. In these lectures we will try to summarise some of the main features of the detector. We will start from the very basic concepts and gradually advance to a more general treatment. We will focus our discussion on the solutions adopted in the second generation of instruments, by means of which now the era of the gravitational astronomy has started. In the final sections, in view of the development of the future gravitational wave detectors, we will discuss how to reduce two main noise sources: the thermal noise via the use of cryogenic techniques and the readout noise via the strategies conceived to circumvent the quantum limit of the detector.