On the classifying space and cohomology of Thompson's sporadic simple group D. J. Benson To John Thompson on the occasion of his 70th birthday. \begin{abstract} This paper examines the classifying space of Thompson's sporadic simple group. Two theorems are proved. The first gives a homotopy colimit decomposition for the $2$-completion of the classifying space. The second shows that the mod two cohomology contains a copy of the Dickson invariants of rank five, over which it is finitely generated as a module. \end{abstract}