Assessment of the surface tension of low-energy solids by means of easy to perform contact angle measurements would be very attractive. Two different approaches are frequently reported to be very promising in this respect. We have evaluated these approaches using mainly apolar surfaces, which present the simplest case possible. The "equation of state" approach, which uses a single parameter, correctly predicts the results on FC722 (perfluoropolyacrylate) and FEP (poly(tetrafluoroethylene-co-hexafluoropropylene)), but shows systematic deviations on the surfaces of octyltrichlorosilane (self-assembled on glass) and PE (polyethylene). The "surface tension components" approach uses three parameters. The surface tension is split into a van der Waals component, a Lewis acid component, and a Lewis base component. The determination of the surface tension of apolar surfaces yields reasonably consistent results when using a large set of contact angle data. However, the present results indicate important differences with previously reported values of the van der Waals components of some fluids, i.e., dimethyl sulfoxide (DMSO), formamide, diiodomethane, and 1-bromonaphthalene. The latter two appear not to be apolar in nature when obtained from measurements on FC722 or FEP. The difference with previous reports is due to a discrepancy between the contact angles of diiodomethane and 1-bromonaphthalene measured on either FC722 or PE. It is concluded that neither the "equation of state" nor the "surface tension components" approach can account for all experimental results. The present use of contact angle measurements appears to be limited to the estimation of the surface tension of apolar surfaces, using the "surface tension components" approach.