This chapter discusses the key types of sand dunes and how they can be classified, and assesses the major controls on their development. The processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form. Recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune-field scale rather than at the single-dune scale. Keywords: Classification, Dune dynamics, Dune field, Dune interactions, Equilibrium, Morphology, Sand dune, Sand flux, Self-organization, Wind regime.