The nature of the pairing mechanism in the recently discovered iron-pnictide family of superconductors remains an outstanding issue. To answer this question, it is instructive to know the symmetry of the superconducting energy gap. Low temperature thermal conductivity measurements provide a robust test of the presence or absence of low energy electronic quasiparticles that in turn can be used to characterise the symmetry of the gap function. In this talk, I will review what has been learnt so far from such measurements across several families of iron-pnictide superconductors, including our own recent results on the stoichiometric material LaFePO.