Fog harvesting is an emerging sustainable technology for providing clean water. It involves the collection of water droplets by inertial capture and gravitational shedding on a collector. The need for high collection surface area and low pressure drop necessitates use of a porous collector. This chapter aims to systematically understand the coupled physics of aerodynamics and interfacial fluid mechanics which governs the fog harvesting process at multiple length scales, from tens of metres to a few microns. Key parameters governing fog collection efficiency are identified, which can be divided into geometric and surface properties. Broadly, the geometric properties of the porous collector, like the shade coefficient, pore diameter, and fibre diameter, govern the fog droplet capture, while the surface properties, like wettability and surface roughness, govern the dynamics and shedding of collected water droplets. Several approaches adopted to optimize these two groups of parameters to enhance fog collection efficiency are reviewed. Existing theoretical models and experimental approaches to test the efficacy of such strategies are delineated. Finally, scalability of fog harvesting devices, gaps in understanding the phenomena and future directions of research are discussed.