Based on eight transition criteria, at least two types of transition velocities are identified for the demarcation of the transition from turbulent to fast fluidization. The "critical velocity," U-se , corresponds to the significant entrainment of particles from the bed, beyond which a circulating fluidized bed operation becomes essential. The "transport velocity," U-tr , defines the transition to fast fluidization based on the axial solids concentration profiles. Below this velocity, a distinct interface exists between a top dilute region and a bottom dense region. Above U-tr , the variation of voidage with height becomes relatively smooth. U-tr is found to be a function of measurement location and riser height, as well as gas and particle properties.