Annu. Rev. Astron. Astrophys. 2004. 42: 275-316
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1. INTRODUCTION

One of the most important developments in the field of interstellar gas dynamics during the last half-century was the renewed perception that most processes and structures are strongly affected by turbulence. This is a paradigm shift unparalleled in many other fields of astronomy, comparable perhaps to the discovery of extrasolar planets and cosmological structure at high redshift. Interstellar turbulence and its implications were commonly discussed in the 1950s, but without the range of observations that are available today, the theory of the interstellar medium (ISM) drifted toward increasingly detailed models based largely on a preference for thermal and dynamical equilibria. Standing apart were two subfields that continued to include turbulence as the primary driver of all observations: radio scintillation and cosmic ray transport. The connections between these subfields and the prevailing picture of the ISM were mostly ignored because of the large discrepancy in scales. Consequently, two other small-scale processes got lost in the equilibrium paradigm of the large-scale models: mixing of the elements and molecular chemistry. In this review, we discuss these four subfields in some detail. All have a well-established observational base going back several decades, but because of the complexity of turbulence and the infancy of the relevant theory, all of them are now in a state of rapid evolution.

Our previous review Interstellar Turbulence I (this volume), emphasized observations in the dense neutral ISM and discussed in detail the various theoretical approaches to this field.