‘Microlayers’, the thin (less than 10 μm) films of liquid left behind beneath rapidly-growing steam bubbles at a heated wall, can be a large, and even the dominant, source of the vapour in such bubbles by the time they depart the wall. Given their slenderness (compared to the 1–10 mm diameter of the bubble), and their high aspect ratio, (with radial extents of perhaps order >100 times their thickness), such microlayers are incorporated relatively simplistically in microscopic CFD analyses of bubble growth. However, their role is particularly important because the evaporation of the microlayer generates vapour rapidly, which itself expands the bubble and generates even more microlayer to evaporate. Plainly, a good understanding of the microlayer formation process is desirable. In this paper we present first-principles calculations of the hydrodynamics of the formation of such microlayers. These seem to show overwhelmingly that the determinant of the existence and radial extent of a microlayer is the bubble growth rate, with higher growth rates leading to more flattened and less spherical bubbles, allowing larger microlayers being trapped beneath them. When they are formed, microlayer thickness is then to a degree dependent on the fluid surface tension and liquid viscosity. The need for an extension of these hydrodynamic studies to include a mechanistic self-consistent model of the evaporative depletion of the microlayer is noted.