The expected threshold power (PL−H) required to access H-mode operation on ITER is extrapolated from a multi-machine scaling that is strongly weighted to a dataset of JET carbon wall discharges [1]. Experiments on JET with the Be/W wall show a minimum in PL−H as function of density and a favorable reduction in the threshold power of 30% at medium to high densities [2]. The carbon wall threshold can be recovered with nitrogen injection. An increase in the threshold power by up to a factor 2 and a reduction of the density at which the minimum occurs was found when the outer strike point position was moved from the horizontal to the vertical target. The existence of a critical min(Er,dia) is consistent with the threshold data from each of the divertor configurations but does not explain the differences between them. The observation of a strong asymmetry between the inner and outer divertor that develops during a power ramp is correlated with the L-H threshold in the high density branch and indicates a direct effect of the scrape off layer or divertor conditions on the threshold. This mechanism also provides a qualitative explanation for dithering L-H transitions.