Optical, but especially laser techniques, have been largely applied over the past decades to characterize complex multiphase flows. The strength of optical diagnostics lies in the null or limited invasion and the typical high space and time resolution can be achieved also in very hostile combustion environments. Nowadays, the use of increasingly sophisticated theoretical models, the utilization of new generation fuels and combustion technologies require an unprecedented spray diagnostics capability. In this chapter, the optical techniques for the analysis of complex reactive sprays are revised. Emphasis is placed on the fact that droplets in instationary sprays can be thought as microscopic reactors where physics, chemistry and fluid dynamics simultaneously interact in a complex way. Their characterization represents an extraordinary challenge for experimentalists. On the one hand, it requires a careful analysis of the basic principles of optical diagnostics; a crucial point is the identification of the limits of applicability of optical techniques in practical systems, due to the underlying theoretical assumptions. On the other hand, it demands the cautious application of optical methods due to the significant transformations underwent by fuels during evaporation and combustion in spray flames. Both the themes are developed in the chapter.