The current study examined the stability of temperament over time. Observers and mothers rated child behavior at eight timepoints across three assessment waves (8, 15, and 30 months of age). Internal consistency reliability of aggregates of the eight observer reports and eight mother reports were high. When considering single timepoint assessments, stability of temperament between assessment waves was generally low to moderate in magnitude. When the eight observations were aggregated, rather than when single timepoints were considered, between-wave stability of temperament increased. Mother reports of temperament were generally more stable than observer reports. Results have important methodological implications and suggest multiple observations of child behavior within a single developmental period are necessary to evaluate the stability of temperament over time.