Fibrous filters are widely used in manufacturing and separation industries to remove aerosol particles produced in the production process. However, due to the complex internal structure of fibrous filters, there are still some problems to be investigated. In the first part of this review, the different theoretical analyses of the flow field distribution around the fiber is summarized. In a comparison of the various theoretical models of flow fields with the simulation results, the Kuwabara-Happel flow field model has the best applicability. The theoretical models of single fiber filtration by five mechanisms: diffusion, interception, inertial impaction, gravity settling, and electrostatic effects are summarized. One of the other outstanding questions is the effect of fibrous microstructure characteristics on filtration efficiencies, such as the composite layer structure, the heterogeneity of the fibrous filters, and the fiber orientation. Finally, critical gaps in the literature and necessary directions for future investigations are also given.