In this paper, flow boiling phenomenological differences between micro- and macroscale channels are discussed. Analyses of flow pattern results are performed in order to identify the main differences between micro- and macroscale phenomena. Based on these analyses, important flow pattern differences between micro- and macroscale behaviors are identified, and new phenomenological transition criteria based on flow patterns are presented. These criteria are based on the existence of stratified flow and on the degree of uniformity of the liquid film along the tube perimeter during annular flow.