This paper reviews methods for the prediction and correlation of Newtonian viscosity for pure components and mixtures of dilute gases, dense gases and liquids, focussing on those which are suited for practical engineering use. The methods reviewed were chosen because they are well known and accepted or appear potentially promising. They are categorized as theoretical, semi-theoretical or empirical and further distinguished as predictive or correlative. Brief derivations with relevant equations and discussions on limitations and reliability of results are presented. In addition, the applicability and average deviations for each method are tabulated, with the recommended methods clearly stated. Furthermore, some gaps in viscosity prediction/correlation are identified and promising approaches are discussed.