This book is a detailed treatment of volume and surface force‎-densities and mechanical stresses in electrically and/or magnetically polarized bodies‎.‎ The classical approach applies equally well to the electric and/or to the magnetic case‎.‎ The issue of computation of force densities in materials is still a controversial one‎,‎ but it is very important in many practical applications‎.‎ These include the design of electric machines and various power apparatus‎,‎ permanent magnet devices‎,‎ and piezo‎-electric actuators and sensors‎.‎ By combining electrodynamic theory‎,‎ continuum mechanics and classical thermodynamics‎,‎ important and reliable formulas for force densities are derived and settled‎.‎ In particular‎,‎ the well‎-known controversy between the Helmholtz and the Kelvin formulas for force densities in linear fluid dielectrics is analyzed in detail in the light of existing experimental results‎.‎