This chapter subdivides the hydrocarbon migration into primary, secondary, and tertiary migrations. These are described as a multiphase fluid flow driven by petroleum fluid potential gradients. The primary migration represents the release of generated hydrocarbon molecules from the kerogen matrix when the sorptive capacity of the matrix is exceeded, often called expulsion by pressure-driven movement through the source rock matrix and transient microfractures. In the case of oil, the secondary and tertiary migrations represent a longer-range flow from source rock to reservoir and remigration from one accumulation to another, respectively. It takes place through a combination of carrier beds, faults, and fractures driven by the balance between fluid potential gradients that are created by buoyancy force, hydraulic gradient, capillary pressure and frictional resistivity force. Description of each force contains mathematical formulations. The secondary migration is described as including separate phase flow, diffusion, solution, and dissolution of gas in oil and water and chemical cracking. The discussion is supported by case studies from the literature.