Displacement flow and mixing of fluids occur in many operations present in the construction and completion of oil & gas wells e.g. primary cementing, drilling, gravel-packing, hydraulic fracturing. Over the past many years oil prices have increased significantly as a result of a diminishing supply of oil and gas from easily accessible reservoirs coupled with an increased demand due to population and consumption growth. Inevitably, there is a tendency towards deeper wells offshore and towards more challenging wells onshore. Many oil and gas wells required are long and are inclined anywhere from vertical to fully horizontal. This range of wells brings new challenges, a wider range of fluids used in the processes and additional higher risks as operational failures can become highly expensive and catastrophic. Different approaches were employed in an integrated fashion to tackle the problem in this book namely experimental, computational and analytical. The aim is to understand the displacement flows that are present in well operations and thus improve the well construction process.