The multidisciplinary field of Microwave-Photonics is at the convergence of photonic and microwave or even wireless technologies. In this context, the processing, conversion and transmission of microwaves, as well as high data rate digital and wireless signals, today use broadband optical techniques with many applications. However, the frequency band available is frequently changing with up-to-date commercial standards, and it is necessary to up- or down- convert the frequency of the electrical signals using a suitable optical approach for MIXING. This paper intends to give an overview on the optical techniques used for frequency conversion, i.e. MIXING, and gives the advantages and drawbacks of all techniques presented.