This study aimed at exploring the understanding of concept inventories for classroom assessment by physics tutors and pre-service teachers in selected diploma teachers' training colleges in Tanzania. The study employed a qualitative research approach and a case study design. Eight physics tutors and 20 second-year pre-service physics teachers from two selected diploma colleges were involved in the study. Data were collected using tutors' interviews and pre-service teachers' focus group discussions. Analysis of data was carried out by using the deductive thematic analysis technique. The findings revealed that these physics tutors and pre-service teachers understand concept inventory as a question-and-answer assessment strategy and as a criterion-referenced test. It was also revealed that both groups were not familiar with the format and examples of concept inventory tests found in physics. Moreover, the findings indicated that participants were conducting diagnostic assessments by using other forms of assessment tools apart from concept inventories. The assessment practices were dominated by traditional assessment methods such as written tests and examinations. The study concluded that assessments through concept inventories remain a big challenge to the participants. The study recommends providing in-service training for physics tutors on the use of concept inventory for effective assessments of pre-service teachers and training the latter on how to use concept inventories.