The most important aspects of passive microwave remote sensing has been explored both from theoretical and for operational point of view. The chapter does not rigorously treat the physical principles of PMW remote sensing, but uses theory as a reference point to correctly interpret and describe satellite observations. For this reason, the first sections were contributed to focus the attention on two fundamental themes that must be taken into account when using microwave radiometers: surface emissivity and radiation extinction processes. Compared to optical and IR wavelengths where surface contribution varies between 0.80 ÷ 1.00, microwaves are very susceptible to changes in surface conditions. Over ocean, the substantially stable emissive surface ensures that microwave soundings of atmospheric parameters are quite consistent within a strategy of rainrate retrieval. Over land areas, the passive microwave observations yield to significantly less quantitative measures of rainfall because the effects of surface emission variability can drastically affect measurements and consequently the retrieved products. Those surface effects are more marked in the case of