\title{Stratification and $\pi$-cosupport: Finite groups} \author[Benson, Iyengar, Krause, and Pevtsova]{Dave Benson, Srikanth B. Iyengar, Henning Krause \\ and Julia Pevtsova} \begin{abstract} We introduce the notion of $\pi$-cosupport as a new tool for the stable module category of a finite group scheme. In the case of a finite group, we use this to give a new proof of the classification of tensor ideal localising subcategories. In a sequel to this paper, we carry out the corresponding classification for finite group schemes. \end{abstract} \keywords{cosupport, stable module category, finite group scheme, localising subcategory, support, thick subcategory} \subjclass[2010]{16G10 (primary); 20C20, 20G10 20J06 (secondary)} \date{May 24, 2015}