With the recent discovery of superconductivity in iron-based superconductors, interesting questions are posed regarding the pairing symmetry of the superconducting state, which is intimately linked to the structure of the superconducting gap. In my talk, I will present an overview of London penetration depth measurements taken on these materials with the use of a tunnel diode resonator (TDR) circuit in order to address the question of the gap structure. Emphasis will be placed on the so called “122” class of these materials of which large, high quality single crystals are available. The details of the superconducting gap structure, in particular the presence of nodes, has been inferred from the doping evolution of the experimentally determined power law behavior of the penetration depth. Comparisons will be made to penetration depth measurements in the LiFeAs system, believed to be an iron-based superconductor in the clean limit, and also to thermal conductivity measurements.