Inquiry-based education has been part of innovative science teaching for the last few decades. With the competence orientation now underlying many national curricula, one of the emerging questions is how the development of student competences can be fostered in the context of inquiry-based science education. One approach to supporting students in their learning is formative assessment, which is, however, not frequently used in a structured way in daily teaching practice. The aim of this study therefore is to explore what kinds of measures might support science teachers in implementing formative assessment activities in their inquiry-based education. For this, firstly, we investigated the challenges that occur on a classroom level when using formative assessment methods in inquiry-based science education from teachers' perspectives. Secondly, we analyzed the teachers' suggestions on measures of support. Based on the respective results, this paper discusses implications for an implementation of structured formative assessment in inquiry-based science education to enhance student competences.