Bone age determination is one of the important radiological methods of diagnosis. However, unfortunately considerable misunderstanding is seen among physicians and even radiologists. This is due to a general lack of knowledge concerning basic concepts, normal variation and the areas choosing for X-Ray. Most of errors are attributable to ignorance of the expected range of normal bone development. A well-known atlas of wrist and hand by "Greulich and Pyle" is widely accepted because of its simplicity and low radiation exposure, but the bones of these areas do not necessarily reflect development of the skeleton generally. This method is more abused by referring physicians when they limit the X-Ray to the wrist region, which considerably reduces accurate assessment. In this paper, different aspects of subject, including assessment and clinical use of neonatal bone maturation, such as hypothyroidism, low birth weight babies, and respiratory distress syndrome are also dicussed.