Coming to know about the physical world is a long process that begins early in infancy and is not completed for years. The child's contact with the physical world comes via the senses, and thus the first set of questions has to do with the quality of the infant's perceptual capabilities. Perception is not the end of the matter, though – the infant and child also have to acquire the skills to engage in motor and locomotor interaction with the environment. Throughout, the child gradually gains the ability to conceptualize the physical world through thought, rather than just acting upon it. A large and important part of this growing ability to conceptualize is the perception and understanding of the spatial aspects of the physical world. In Part I we address each of these major issues in separate chapters.