Most conceptualisations of the bottom billion assume that "the poor" are a minority group in a state of continuous dependency, identifiable by region and demographic. Using a flow analysis (inflow and outflow) of poverty, rather than a stock analysis, we explain why poverty is more appropriately understood as a dynamic, with the majority of people flowing in and out of poverty for short durations. Distinguishing between structural and transitory poverty gives rise to a focus on the identification of multiple constituencies in the wider population including the permanently poor; sometimes poor; and non-poor. External shocks, including economic and environmental shifts, and risks such as ill-health, can affect any individual, household, or population in a non-predictable way, and can lead to loss of livelihood and a descent into poverty for various durations. At any point in time the bottom billion is made up of a blend of both transitory and structural elements with the former reflecting poverty as a risk for a much wider population than is often assumed.