Particle filtration efficiency is governed by mechanical mechanisms, i.e. interception, impaction and diffusion, as well as by electrostatic mechanisms, i.e. electrostatic attraction force. The mechanical mechanisms are determined by the surface area of fibers on the web. Therefore, reducing fiber diameter improves filtration efficiency. However, filters have a low filtration efficiency for very small particles by mechanical mechanisms. The filtration efficiency is greatly improved by electrostatic charging of fibrous materials. Charging theory and charging techniques will be described. The improvement of filtration efficiency by electrostatically charging the meltblown polypropylene (PP) webs using the techniques developed at the University of Tennessee as compared with other techniques will be discussed.