More than half of the volume of the universe is made up of under-dense regions. This means that light from distant objects spends the majority of its journey to us in under-dense regions. Therefore the analysis and classification of such under-dense regions (in both LCDM and other models) will be of great use in the era of precision large scale structure surveys we are about to enter. I will discuss methods that can be used to construct such catalogues, as well as listing various potential pitfalls that can (and have) occur(ed) when analysing voids. Regarding potential cosmological applications, I will make specific mention of the integrated Sachs-Wolfe effect, where some results have been found that *could* be pointing to new physics (or they could just be a fluke).