In this presentation for the GeoGebra Global Gathering, we discuss the role of a content-specific, interactive, scenario-based assessments of mathematics teachers' knowledge that incorporate virtual materials developed in GeoGebra, used in mathematics teachers’ professional development. The process of development and implementation of the pre and post assessments, as well as the nature and the details of the professional development (PD) are described to the session participants. The focus of this paper is on PD and use of Geogebra-based instruments of teachers' Pedagogical Content Knowledge in geometry at the secondary level.