Landmarks are the changes in certain physical and mental capacities (such as to walk and the understanding of the language) that mark to the end of a period of development and the beginning of another one. For theories of the stages, landmarks indicate a phase transition. Studies of the accomplishment of many tasks of development have established typical chronological ages associated with development stages. Nevertheless, a considerable variation in the accomplishment of the stages exists, even between the children with trajectories of development within the normal rank. Some landmarks are more variable than others, for example, the indicators receptive of the speech do not show much variation between the children with normal hearing, but the expressive landmarks of the speech can be very variable.

A preoccupation common in the infantile development is the delay of the development that implies to a delay in the specific capacity of the age of important landmarks of the development. Prevention and early intervention in the delay in the development are important subjects in the study of the infantile development. The delays of the development must be diagnosed by comparison with the variability characteristic of a landmark, not with respect to the average age of the profits. An example of a landmark would be the coordination eye-I flow, that includes an increasing capacity of the boy to manipulate objects in a coordinated form. Greater knowledge of the specific stages by age allows to the parents and other people to make a pursuit of the suitable development.