  We develop a semiclassical theory for the dynamics of electrons in a magnetic
Bloch band, where the Berry phase plays an important role. This theory,
together with the Boltzmann equation, provides a framework for studying
transport problems in high magnetic fields. We also derive an Onsager-like
formula for the quantization of cyclotron orbits, and we find a connection
between the number of orbits and Hall conductivity. This connection is employed
to explain the clustering structure of the Hofstadter spectrum. The advantage
of this theory is its generality and conceptual simplicity.
