The origin and propagation mechanisms of fluctuations in the cosmic rays observed by instruments on the ground, in the stratosphere, and in space are discussed. Recent experimental results on fluctuations at frequencies from 10$^{-3}$ to 10$^{-6}$ Hz are discussed. The primary cause of the statistically significant fluctuations in the cosmic rays is the scattering of charged particles of the cosmic rays by random inhomogeneities of the interplanetary magnetic field.