Advances in the theory of spin fluctuations have resulted in a remarkable synthesis of the microscopic spin dynamics and the bulk properties of weak ferromagnets and exchange enhanced paramagnets. In the case of systems like YMn2 and β-Mn, which display antiferromagnetic spin fluctuations, it appears that topological frustration is an important extra factor, leading to quantum spin liquid behaviour. Inelastic neutron scattering and μSR have important and complementary roles in characterising the spin dynamics in these intriguing magnetic materials.