This document discusses the development of occlusion from birth through the primary and mixed dentition periods. It describes the key stages and changes that occur during predental, deciduous dentition, and mixed dentition periods. During the deciduous dentition period, the primary teeth erupt in a spaced arrangement to allow for proper alignment of the permanent teeth. The terminal plane between the maxillary and mandibular second primary molars is an important reference point. A modified Angle's classification is also described for assessing occlusion during the primary dentition stage.