Abstract: I will examine the symbiotic relationship between two torsion radicals, one of Martsinkovsky-Russel and the other of Bass. The latter is just the kernel of the canonical bidualization map, whereas the former requires a functorial approach, which I will explain in the lecture. It is the largest subfunctor of the Bass torsion preserving filtered colimits. As such, it behaves better on infinite modules. The two radicals coincide on finitely presented modules. The MR torsion admits a dual concept, which we call cotorsion, the duality being effected by the Auslander-Gruson-Jensen transform. This will be an expository talk. No prior experience with functor categories is needed.