The Toddler Behavior Assessment Questionnaire (TBAQ) was constructed by an iterative process of item generation intended to ensure content validity, by repeated item analyses focused on internal consistency and discriminant properties, and by scale revision. During the construction and initial validation processes reported in this article, data from 1,012 records were utilized. Internal consistency reliability estimates typically exceeded .80 for each scale. Evidence for convergent validity with other temperament questionnaires and for longitudinal stability was also obtained. Besides yielding a promising instrument, this assessment research has conceptual ramifications. For instance, components of negative affectivity (anger proneness and fearfulness) were independent, and item analyses suggested that shyness and other fears were independent as well. Consistent with most current views of temperament, the TBAQ temperament scales revealed some relationship and/or contextual specificity, as exemplified by the finding of only moderate parental agreement. The rank ordering on most temperament dimensions was impressively preserved from age 12 months, when the Infant Behavior Questionnaire (IBQ) was used, to age 18 months, when the TBAQ was used (especially when subtle differences between the IBQ and TBAQ were taken into account). Most of the analyses benefit from replication.