Primary and secondary migration of hydrocarbon are generally recognised among petroleum geologists. The former is defined as an expulsion of hydrocarbon from source to reservoir rocks although its concept is rather complicated in conjunction with petroleum genesis. The latter is the movement of hydrocarbon within reservoir rocks.<BR>On hydrocarbon genesis, opinion seem to have recently converged to the diagenetic theory. As the depth range of petroleum genesis is relatively long, some geologists favor early diagenetic stage of primary migration and the others emphasize late diagenetic stage. Therefore, the mechanisms for primary migration should guite differ between the two ideas, because rock properties considerably vary with depths. In this point of view, the theory of primary migration is not said to be established yet.<BR>The author has studied on the timing of primary hydrocarbon migration in the Japanese oil-bearing Neogene system mainly from the geohistorical aspect, and arrived at the conclusion that its principal timing is during the early mature stage, that is to say, within mature range of 0.5 to 0.7% in vitrinite reflectance Ro, and also that hydrocarbon generated in the late diagenetic stage is not able to migrate any more.<BR>Porosity of muddy sediments during primary migration is estimated to be from 15 to 25%, and media with which hydrocarbon migrates is pore water then exulsed from muddy sediments by compaction.<BR>Although many oil fields are often said to be well explained when assuming late diagenetic stage migration, it is noteworthy many writers recently intends to explain the formation of these oil fields in a light of the theory of early stage migration. Why the timing of primary migration should be seriously discussed is due to not only academic interest but also its technological and economical influence to practical exploration procedures.<BR>On the contrary to the complicated problems on primary migration, secondary one has been a well difined physical process which is quantitatively dealt with through experiments in laboratories. Its main force is bouyancy, and resistant force for secondary migration is capillary pressure. These are relatively easily calculated using simple equations immediately after obtaining unknown physical qauntities related to the equations.<BR>A better understanding of principles of bouyancy and capillary pressure on secondary migration will lead to the effective development of oil fields, and particulary to the discovery of unusual accumulations such as permeability traps.