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The wavy structure of liquid film in downward annular gas-liquid flow with and without liquid entrainment is examined for a wide range of gas velocities using high-speed laser-induced fluorescence technique. It is shown that the wavy structure is always represented by two types of waves, long-lived and short-lived waves with the latter always generated at the back slopes of the long-lived waves. Main regularities of short-lived wave generation and evolution are also described.Annular flow represents the combined flow of liquid film along the channel wall and high-velocity gas stream along the center of the channel. At high gas and liquid flow rates liquid droplets are entrained from the film surface into the core of the gas stream. Entrainment contributes significantly to integral heat and mass transfer, and its modeling entails high practical interest. It is now acknowledged that two types of waves coexist on the film surface in case of entrainment: long-length disturbance waves with amplitude several times higher than the average film thickness and small-scaled ripple waves. According to high-speed video observations of Woodmansee and Hanratty, 1 entrainment occurs due to "acceleration, lifting and subsequent scattering of ripples on disturbance waves crests."For physical modeling of entrainment phenomenon, detailed information on the origin of the two types of waves is required. At present ripples are considered to be omnipresent on the film surface in the presence of turbulent gas stream, even for very low film Reynolds numbers. With liquid flow rate growth disturbance waves appear and entrainment occurs. 2,3 Existence of critical Reynolds number is the characteristic feature of entrainment phenomenon. At Reynolds numbers lower than this value entrainment does not occur irrespective of gas stream velocity. 2,3 This feature is usually explained by the absence of disturbance waves in regimes without entrainment ͑hereinafter such regimes will be referred to as "no-entrainment regimes"͒. So, the contemporary notion of the wavy structure of annular flow is as follows: when entrainment occurs, there exist two independent types of waves on liquid film surface-disturbance and ripple waves-and when entrainment is absent, only ripple waves exist.Some attempts have been made to obtain a more detailed representation of the wavy structure in annular flows. Sekoguchi et al., 4 using a complicated set of conductivity sensors for spatiotemporal investigation of liquid film thickness, found "ephemeral waves," which sometimes appear on the back front of disturbance waves and move slower than the latter. Ohba and Nagae 5 observed one more type of waves in the narrow region near the transition to entrainment in upward annular flow. Those waves, which they called "ring waves," were shorter than disturbance waves and wider than ripples. But common practice is to divide waves into disturbance waves and ripples and to study them separately. 6,7 The ripples are known to be short lived and coherent only for short ͑about several wavele...