This paper analyzes and discusses the informal assessments of newly qualified teachers (NQTs) that are performed in relation to the mandatory formal assessment outlined in the recently introduced teacher registration reform in Sweden (TRR). Questionnaire data was collected in three municipality clusters. 11 semi-structured interviews with principals were conducted in two municipalities. These were analyzed to determine whether and how principals used informal and interpersonal sources of information about NQTs’ competences. The school culture, together with the use of formal and informal assessment, influences a principal’s final assessment of an NQT. The use of informal and interpersonal sources of information for assessment fit the school context in ways that a standardized reform cannot match.