A review of the current state of the kinetic theory of acceleration and propagation of energetic (thermal and non-thermal) electrons during solar flares is presented. The classical models of a thick-target and their further development are considered in detail — models that take into account the effect of the reverse-current and the acceleration of electrons in collapsing magnetic traps. Analytical solutions of the corresponding kinetic equations are found. Based on these solutions, the characteristics of the bremsstrahlung X-ray emission generated by energetic electrons are calculated. The obtained results are compared with modern data of high-precision observations of flares. The degree of X-ray polarization is calculated, and the possibilities of measuring it in future space experiments are discussed.