Abstract: The main objective of this review study is to present the current state of available research results and understanding on the effects of emulsions on flow behaviours and flow properties, as well as common factors affecting the stability of emulsions. Emulsions occur in many areas of daily life and industry settings but this review study only intends to discuss emulsions associated with upstreams crude oil production, particularly the oil pipelines transporting system. In the transporting pipelines, there are possibly water-in-oil emulsions, oil-in-water emulsions and multiple emulsions. Generally, adverse effects caused by emulsions are related to higher viscosity, larger pressure drop and more oil-water separation difficulty. This correspondingly results in the need for higher pumping capacity and larger capacity of vessels or pipelines. The adverse effects are significant only when the emulsions are stable, so this necessitates the study to manipulate the stability of emulsions.