More than 75,000 articles have been published on internalizing and externalizing problems. To advance clinical and research applications of internalizing/externalizing concepts and data, our objectives were as follows: to provide an overview of recent research on internalizing/externalizing problems assessed at ages 1½ to 18 years; to identify issues raised by methods for assessing such problems; and to develop recommendations for more precise, consistent, informative, and productive assessment of such problems.