CT and MR imaging of the paranasal sinuses in infants and children reveal a spectrum of findings associated with the normal pneumatization process, both inside the sinus cavities and in the adjacent marrow spaces. These normal findings must be understood and recognized so that CT scans and MR images may be accurately interpreted. If such normal developmental radiologic findings are not appreciated, misinterpretation may occur and lead to inappropriate treatment. In this pictorial essay, the normal process of pneumatization for each paranasal sinus group will be described from the first stages of the process to its completion. CT scans and MR images will illustrate the range of normal radiologic findings associated with the developmental process, with emphasis placed on the types of findings that, although normal, create potential interpretive difficulties.