Commonly used models for predicting the flow patterns and flow pattern transitions are established for gas-liquid flows in normal-size channels (D > 0.5′′). Those are generally found to predict poorly experimental two-phase flow pattern data in minichannels. In this study, the effect of the channel diameter on the mechanisms leading to flow pattern transitions are reexamined in an attempt to identify the governing phenomena involved in two-phase flow in small-diameter channels. Accordingly, appropriate mechanistic models are suggested and compared with experimental flow pattern maps available from the literature. These models also indicate the controlling dimensionless groups and the critical values associated with the various flow pattern transitions. The analyses also suggest criteria, in terms of dimensionless Eotvos number, Eo_D, that indicate when the conventional models must be substituted with the minichannel models. The reasons for the disappearance of the stratified flow in mini- and microchannels are elaborated, and the various mechanisms leading to the establishment of annular flow as the basic flow pattern in low Eo_D systems are discussed.