An explanation for the LH transition through the electromagnetic mechanism: suppression of drift wave turbulence by the skin effect, is offered. In dimensional space, the bifurcation is attributed to the involvement of two parameters in the LH transition physics: ρ* and β/ν*ρ*2 (the latter scales as aT2 in physical parameters). Maximum of the diffusion coefficient, corresponding to the LH transition, is reached when the collisional skin-depth Δcoll.skin =c2/8σν (ν- drift frequency) equals the characteristic radial displacement Δ of the drift turbulence. The same criterion can also be presented as: D⊥ ≡ Δ2ν = c2 /8σ - the condition for equal rates of the plasma diffusion into the magnetic field and the diffusion of the magnetic field into the plasma. The analysis yields the combination Te13/8(Te + Ti)3/8 / B(q/r)1/4 (scales as [β/ν*ρ*2/3]4/4in dimensionless parameters) as a critical parameter for the LH transition, for the case of k⊥ρs = const the scaling for the wave vector of the drift turbulence. This threshold parameter should be applied near the separatrix position. A requirement that the collisionless skin-depth must be smaller than the radial displacement of the drift fluctuations in the Lmode, which is necessary for turbulence suppression, determines the threshold β for the LH transition. The proposed mechanism for the LH transition clarifies the R-dependence of the Hmode power threshold: Pthres ~ R1.75 scaling is predicted, with Pthres < 70MW for ne = 5 x 1019 m–3 in ITER. The critical parameter for the LH transition, together with dimensionless parameters characterising pressure gradient and resistivity, create the set of similarity parameters describing ELM behaviour. The scaling for the separatrix density normalised to the Greenwald density limit ne,sep/nGW with the machine size and toroidal field which ensures &quot;similar&quot; ELM behaviour can thus be obtained. For the fixed similarity parameters, the analysis yields weak (~R1/4) but favourable dependence of ne,sep/nGW on the major radius. In recent experiments on JET and other machines, the degradation in the edge confinement associated with increased ELM frequency was found to be responsible for the density limit in high power Hmodes. Owing to the approximately R1/4 dependence, an excess over the Greenwald limit, ne/nGW, by about 30% higher in ITER compared to JET for "similar" conditions (q, ne.sep/ne , separatrix zeff and the Te/Ti ratio, wall conditions, the use of pellets etc.) in ELMy Hmodes is predicted. This is with the provision that a limit on the central density, related to mechanisms in the plasma core, is not encountered.