Makerspaces are recent additions to schools and have been promoted as a means of developing STEM knowledge and skills. According to literature, the practical nature of making supports deeper engagement with STEM concepts and enhances development of STEM capabilities such as creativity, critical thinking, problem solving and collaboration. However, to date, limited empirical work has been completed investigating STEM learning in school makerspaces. This article reports outcomes from a study of 24 classroom makerspaces, where 5-8-year olds used 3D printing technology to design and develop artefacts responding to different problems, needs and opportunities. Findings were mixed, with evidence supporting makerspaces as effective for STEM skill and disposition development but more limited in their capacity to build STEM knowledge, unless this was explicitly identified and targeted by teachers. This paper questions assumptions about makerspaces as implicitly effective for STEM knowledge-building, arguing that teachers must specifically target conceptual outcomes in planning and teaching if makerspaces are to be effective for this purpose. Also, findings suggest the need to rethink how makerspaces contribute to holistic STEM literacy development, moving beyond current perspectives focused on learning "about" STEM, to one where makerspaces are viewed as "epistemic environments" beneficial to knowledge-building, "of" STEM. Findings will be of value to educators considering makerspaces as a component of STEM curriculum and infrastructure development.