Surface treatments introduce chemical modifications to the fiber surface that affect the surface free energy (SFE). This is done either with the obvious aim to change the wetting behavior, or to affect related properties, such as, e.g., adhesion phenomenon, surface conductivity, adsorption of proteins, etc. On planar substrates, the measurement of contact angles of specific liquids and making use of formalisms such as Neumann or Owens-Wendt equations is a commonly used approach to determine the surface free energy. It is to be observed that this direct approach is often and lightheartedly applied to porous and textured samples, such as textiles, too. The geometry of a textile is extremely complex and defined by the topography of the fiber, the construction of the yarn, and the construction of the fabric. In addition, polymer fibers may be porous and take up water from the environment. Accordingly, wetting is the result of simultaneous spreading on a rough surface, penetration, and capillary motion in the multi-porous system. Therefore, the critical consideration of any analytical method for wettability measurements cannot be overemphasized, and the present paper is meant to critically discuss the pros and cons of various methods common to the textile researcher. It can be summarized that contact angles can be useful for comparative measurements on hydrophobic samples, while the established drop penetration tests characterize the effects of fabric finishing, fiber surface modifications, etc. with limited quantification. By no means can these test be used to derive the SFE, and in all cases it is essential to avoid accidental distortions of the fabric. The single fiber micro-Wilhelmy method can be regarded as the only reliable method to obtain advancing and receding contact angles. (C) Koninklijke Brill NV, Leiden, 2011