This review evaluates the present state of the art on backmixing in gas-liquid and gas-liquid-solid reactors. A brief outline of numerous techniques for measuring residence time distribution (RTD) of various phases in a multiphase reactor is presented. This is followed by a brief description of differential and stagewise models for characterizing backmixing from RTD measurements. Both simple (that is, single-parameter axial dispersion model) and more complex (that is, two-, three-, or four-parameter models) models are evaluated. Backmixing characteristics of various gas-liquid columns such as trickle beds, spray columns, mechanically agitated columns, plate columns, fluidized bed columns, etc., are subsequently evaluated. The performance of a bubble column under various reaction conditions is analyzed. Criteria for the elimination of backmixing in packed-bed reactors are presented, and the effect of backmixing on the multiple steady states in a gas-liquid reactor is briefly reviewed. Finally, the scale-up problems associated with gas-liquid reactors with various degrees of backmixing and the recommendations for the future work in RTD and macromixing models are outlined.