There is a need to assess whether the resources invested in building trauma competence in services actually lead to higher levels of competence. For this purpose, we have developed an instrument called the Trauma and Development Education Monitor (TANDEM). The present article presents the process behind developing the instrument and an initial study of its psychometric properties. The tested version consisted of 59 items distributed across the domains of readiness, agency, reflexivity, knowledge, practice and culture at work. Based on responses from 415 professionals in relevant services, concept and criterion validity was examined through reliability analyses, factor analyses and regression analyses. Based on the analyses, the tested version was reduced to 54 items in the final version, distributed across the same six domains. As dimensions, the domains generally showed good scale reliability, and for the final version factor analyses confirmed a one-dimensional model for all domains. The instrument discriminated between respondents who reported more versus less previously received trauma competence building and showed specificity as a measure of trauma versus more general competence. The results indicate that TANDEM has the potential to become a useful tool in endeavours to implement trauma competence in services, as well as in research activities in this field.