The number of reflectivity studies has largely increased in the last years so that this technique is nowadays well developed using various x-ray sources ranging from sealed tubes and rotating anodes to last generation synchrotrons with setups adapted to liquid or solid surfaces [1,2]. This chapter is focussed on experimental solid surface studies. Let’s just mention that solid surfaces allow more flexibility than liquid ones since they can be oriented in any direction without deviating the incident beam. In addition, some other questions about resolution with long range correlations on liquid surfaces are generally avoided with rough solid surfaces.