  In this course we review the theory of incompressible homogeneous turbulence
at an elementary level, and discuss the similarities and differences expected
in the compressible case, relevant to the interstellar medium and molecular
clouds. We stress that a general definition of turbulence applicable to the
compressible case should not rely on the Kolmogorov $k^{-5/3}$ spectrum nor on
an energy cascade from large to small scales. Instead, we discuss the various
possibilities for the energy spectrum of compressible turbulence, which
numerical simulations suggest should be $\sim k^{-2}$, and the nature of the
cascades, if at all present. We then discuss issues concerning molecular clouds
which are likely to be directly related to turbulence, such as cloud formation,
cloud structure, and cloud support against gravity.
