Gas-solid micro fluidized beds have unique characteristics such as bed pressure drop overshoot, delayed minimum and bubbling/slugging but advanced turbulent fluidization, as well as negligible gas back mixing. The data covering a wide range of operating conditions reported by various researchers are summarized, and analyzed in an effort to gain in-depth understanding about these characteristics. Based on the analysis, an approximate quantitative criterion to distinguish micro and macro fluidized beds is proposed, and the boundaries between non-fluidizable and fluidizable zones are defined. In micro fluidized beds, gas-solid flow can be situated in particulate, bubbling, slugging and turbulent fluidization regimes based on superficial gas velocity, ratios of D-t/d(p) and H-s/D-t. Hydrodynamic characteristics of these flow regimes are discussed, and the correlations for U-mf, U-mb, U-ms, and U-c are developed based on analysis of experimental data. Finally, generalized fluidization maps applicable to micro fluidized beds are proposed based on the developed correlations. Through the analysis, some uncertainties and even contradictories among the reported data are identified, which would guild future studies to bring the micro fluidized bed technology forward. (C) 2020 Elsevier Ltd. All rights reserved.