Two types of clustering of adsorbed molecules on homogeneous surfaces are considered : one is two-dimensional and the other is three-dimensional (droplet-like). The first is likely to occur when the interactions between two adatoms are sufficiently weak compared to those between an adatom and adsorbent; the second is probably associated with cases in which the above condition is inverted. The main conclusion is that at a critical pressure, for which the Langmuir isotherm predicts a low occupation of the surface, a jump from low occupation to complete coverage can occur. The complete occupation corresponds to a monolayer in the first case and to a multilayer or droplets in equilibrium with a mono- or multilayer in the second. Below the critical pressure, the contribution of clusters to the overall coverage appears to be negligible in comparison to that of singlets.