The competition between the catalytic reaction and mass and heat transfer processes is presented with two objective: interpreting laboratory data and predicting full‐scale reactor performance. Internal and external mass transfer at the catalyst scale and gas–liquid resistance are first dealt with. Then heat transfer and possible ignition and extinction phenomena are reviewed. The consequence of these resistances are related to the operating conditions and reactor technology (batch/continuous, fixed bed, suspended catalyst, monolith). Simple experiments and more or less sophisticated calculations are provided to discriminate among the predominant resistances and to select appropriate experimental conditions.