A flow visualization study of the louvered fin geometry, commonly used in automotive heat exchangers was performed. Flow visualization was performed using a dye injection technique with 10:1 scale models. The geometrical parameters, louver pitch, louver angle, and fin pitch were varied to determine their effect on the flow structure. Tests covered louver pitch based Reynolds numbers of 400 - 4000. Data are presented in the form of a dimensionless flow efficiency (defined in terms of the mean flow angle, relative to the louver angle) and Reynolds number. Correlations are developed to predict the flow efficiency as a function of dimensionless geometrical groups and Reynolds number. A discussion of the flow structure is also presented.