Industrial experiences have shown that powders can unexpectedly change from normal powder flow properties to exhibit liquid-like flow characteristics. This change in flow properties, known as flooding, can result in a major loss of a powder's containment. The prime objective of the research presented in this thesis is to develop a method which quantifies a powder's likelihood to flood, and to identify the conditions where the tendency to flood becomes important. A powder is known to exhibit liquid flow properties at high shear rates or when aerated at or above the minimum fluidisation velocity. The interaction of these two factors, however, is not fully understood. A new type of shear cell is developed which enables the measurement of the shear characteristics of an aerated powder. This shear cell is based on Couette geometry, where a powder sample is sheared between two concentric cylinders, while under controlled aeration conditions. Evaluation of the equipment with a variety of powders identifies that the transition to liquid-like flow properties can occur at low shear velocities and at an aeration substantially below fluidisation. The characterisation of a sample of flooded material shows that additional fine particles significantly increases the tendency for that material to flow like a liquid. The effect of additional fine particles on a selection of powders is studied in detail and powders with a narrow particle size distribution are shown to be most vulnerable to flooding. The quantities of fines required before a powder is likely to show liquid-like flow properties can be small, highlighting that the flooding problem can be significantly effected by segregation. The ability to characterise the effect of small quantities of additional fines on the likelihood to undergo liquid-like flow is an important step forward in understanding the apparent random nature of flooding.