The richness and complexity of video portfolios endanger both the reliability and validity of the assessment of teacher competencies. In a post-graduate teacher education program, the assessment of video portfolios was evaluated for its reliability, construct validity, and consequential validity. Although video portfolio facilitated a reliable and valid assessment of teacher competencies, procedures to improve assessment quality were also revealed and are therefore discussed: more explicit grounding of assessment results in the data, peer debriefing, prolonged engagement with the assessment data, cross-checking to find confirmatory or counter examples. (Contains 1 figure and 4 tables.)