This article concentrates on the validity and reliability of portfolio assessment as used in pre-service teacher education. It is not possible to make general pronouncements about the validity of portfolio assessment in pre-service teacher education as there are multiple portfolio applications. The validity depends on the purpose, namely the divers competencies which the course organisers wish to assess with it. Therefore, three categories of competencies and consequently three types of portfolios were distinguished in order to determine the validity of portfolio assessment. For the assessment of teaching and partnership competencies, it is argued that the validity is low due to the roundabout nature of the assessment. On the contrary, the validity of portfolio assessment for learning competencies can be high. The execution of a self-regulated learning process can be accurately assessed using portfolios. The reliability of portfolio assessment is problematic, since it is incapable of fulfilling the classic psychometric requirement of reliability. Nevertheless, provided that the necessary measures are taken, the reliability of portfolio assessment can still be brought to an acceptable level. Five measures are proposed. (Contains 1 figure and 3 tables.)