Various properties of turbulence are discussed with a perspective of their influence on turbulence modelling (TM). The paper considers the relation between spectral properties and TM, the inherent errors associated with TM, the distinction between modelling and numerical errors, local and global processes in turbulence development, the influence of dimensionality on applicability of various models, the potential of LES, and wall proximity corrections. The emphasis is on understanding when and why TM works, not on demonstrative results. (C) 2009 Elsevier Ltd. All rights reserved.