It is well-known that younger children up to the age of about 6 yr. have much difficulty in discrimination between oblique lines in contrast with relative ease in that between horizontal and vertical. This phenomenon is called oblique effect and a large amount of studies were conducted over the past twenty years for determining the causes of such effect. This paper reviewed these experimental studies in terms of the development of the children's strategies in encoding and storing information of oblique orientation in memory. Some recent infant studies revealed that even a baby might have his/her categorical ability of orientation, so, during early childhood, the orientational categories should be much elaborated, and several encoding strategies for non-specific orientation such as oblique should be developed in an appropriate way to each stimulus context. This course of the development seemed to be confirmed on the whole from the present overview of the studies concerned. This confirmation afforded a basis for further discussions on a developmental hierarchy in orientational categories.