Bubble Column Reactors Are Two Phase Gas-Liquid Contactors that Are Widely Used in Chemical, Petrochemical, and Biochemical Industries in Various Processes. a New Hypothesis is Proposed for Hydrodynamic Similarity of Bubble Column Reactors. the Proposed Hypothesis is Evaluated for Hydrodynamic Similarity and Subsequently for Scale-Up of Bubble Column Reactors. Similarity based Only on Global Parameter Did Not Necessarily Ensure the Similar Mixing and Flow Pattern in Two Systems and Hence Scale-Up based on Such Hypothesis Can Be Fatal. State-Of-The-Art Correlations Are Developed to Facilitate the Implementation of the Developed Scale-Up Methodology based on Artificial Neural Network using a Large Databank of Wide Range of Operating and Design Conditions for a Priori Prediction of Hydrodynamic Parameters to Maintain Such Similarity. This is an Abstract of a Paper Presented at the AIChE Annual Meeting and Fall Showcase (Cincinnati, OH 10/30/2005-11/4/2005).