Keywords: eddy diffusivity, conserved tracer, reactive tracer Abstract: Eddies have an important role in transport and dynamical processes in the atmosphere and ocean. They influence the distribution of chemical species and are responsible for driving mean flows. I will discuss the quantification of eddy effects in the atmosphere and ocean. I will focus in particular on two problems. The first is how to quantify the geographic (latitude-longitude) variation of eddy diffusivity of a conserved tracer in such flows. I will describe two different techniques and discuss the implications of the results for an atmospheric and an oceanic case. The second is how to quantify the effects of eddies on the distribution of a reactive tracer. I will take the example of different tracers at the sea surface (temperature, salinity, chlorophyll, etc). Eddy stirring directly influences the distribution of such tracers, but small-scale eddies in the ocean can also influence air-sea interactions and I will describe how this latter effect may be quantified.