  We show theoretically that nonrelativistic nearly-free electrons in solids
should experience a trembling motion
  (Zitterbewegung, ZB) in absence of external fields, similarly to relativistic
electrons in vacuum.
  The Zitterbewegung is directly related to the influence of periodic potential
on the free electron motion.
  The frequency of ZB is $\omega\approx E_g/\hbar$, where $E_g$ is the energy
gap. The amplitude of ZB is determined by the strength of periodic potential
and the lattice period and it can be of the order of nanometers. We show that
the amplitude of ZB does not depend much on the width of the wave packet
representing an electron in real space.
  An analogue of the Foldy-Wouthuysen transformation, known from relativistic
quantum mechanics, is introduced in order to decouple electron states in
various bands. We demonstrate that, after the bands are decoupled, electrons
should be treated as particles of a finite size.
  In contrast to nearly-free electrons we consider a two-band model of tightly
bound electrons.
  We show that also in this case the electrons should experience the trembling
motion. It is concluded that the phenomenon of Zitterbewegung of electrons in
crystalline solids is a rule rather than an exception.
