This work presents the fundamentals and application of transient model-based leak detection and localization technique for crude oil pipelines. The dynamic parameters involved in this model such as pressure, flow and temperature were acquired by SCADA (Supervisory Control and Data Acquisition) system. The characteristic changes in the flowmechanics and thermodynamics along a given length of pipeline were used in detecting, localizing and determining the flow rate of the leak. Measurement of pressure, temperature and flow data at both the inlet and outlet of the pipeline were used in formulating the equations obtained from the inconsistency in the continuity and law of conservation of momentum equations. This model located a leak incident in a horizontal pipeline of length 2000m and diameter 0.3556m carrying Nigeria bonny light crude oil from the Nigeria Petroleum Development Company Limited, Olomoro flowstation into UPS (Ughelli Pump Station) truck line. But the leak located by the model at 1088.12m from the inlet is 11.88m behind the actual leak position of 1100m as discovered during the pipeline leak remedial works.