This chapter provides an overview of the current state of knowledge of the physical phenomena involved in the coupled heat, air and moisture (HAM) transfer in porous building materials. It addresses the mechanisms involved in the hygrothermal transfers. The chapter discusses on the driving potential in porous media and their sensitivity on the transfer. Porous media may be regarded as macroscopically heterogeneous and anisotropic. Coupled heat and moisture transfers in porous media are explored in many scientific areas such as wood drying, hygroscopic textile materials, food and building envelopes, among others. The water vapor permeability is not only dependent on the material and its thickness, but also on the vapor pressure applied on the material. Gravimetric methods are more widely used for the characterization of hygroscopic storage and transfer in porous building materials.