Assessment, an integral part of teaching and learning, is a planned process of identifying, gathering and interpreting information about the performance of students. However, concerns have been raised about how assessment is being conducted in schools and so the aim of this study is to investigate and improve assessment practices used by Grade 12 secondary school Physics teachers in Mozambique, Africa. The study addresses the question of what assessment practices do Grade 12 teachers in Physics in Mozambique apply and how can they be improved? and it adopted a twofold research approach. A Baseline Survey aimed at gaining an overall impression of the assessment practices used by secondary school Physics teachers and an Intervention Study aimed at producing improvements on teacher assessment practices. The preliminary research followed a survey research method, while the intervention applied an educational design research approach. In the survey three questions were investigated: (i) What assessments practices do Grade 12 Physics teachers apply? (ii) What is the quality of the assessment practices? and (iii) How relevant can the assessment practices be for student learning?To address these questions a purposive sample of 12 Physics teachers, four school directors and five educational officers was selected. The survey was conducted in six secondary schools purposefully selected throughout the country and data were collected via interviews, questionnaires, classroom observations and written notes. The Intervention Study was designed to answer the question of how teacher assessment practices can be improved.This phase of the study involved a design, a classroom tryout, and a systematic evaluation of a series of exemplary Physics assessment materials (prototypes) in a context of demonstration experiments. The prototypes were developed for the concepts of force and inertia and their validity and practicality were verified using appraisal by experts, university students, teachers, and students. Classroom tryout was conducted with two teachers and their 62 students in two secondary schools. Baseline Survey findings indicate that the most used assessment practices in schools are paper-and-pencil tests, verbal tests, and homework, while projects, portfolios, and peer assessments are the less frequently used ones. Oral communication during lessons, written work, presentations, notebooks, laboratory work, and ability to solve problems were used as quality criteria for the teachers’ assessment. It was shown that the most frequently assessed student activity was written work, followed by the ability of students to solve problems, while laboratory work was the activity that was never assessed by many of the researched teachers. Another quality criterion used was the type of feedback given by teachers to students, which indicated that teachers were giving expressed (both congratulatory and critical), personal and timely feedback. It emerged that teachers often involve the students in the evaluation of their performance through reflection of assessment results and in addition, encourage students to engage in active learning. Findings from the Intervention Study indicate that (i) teachers liked the presentation and structure of the materials following the Predict-Observe-Explain (POE) strategy and regarded their personal commitment as crucial for achieving the desired experimental results; (ii) students also liked most the POE strategy because it allowed them to develop their own explanations of the observed events and highlighted the role of teachers during the tryout as crucial for the success of the experiments. The main conclusion of this study is that assessment practices undertaken by Physics teachers in Mozambican secondary schools are of poor quality and there is a need for improvement. This must be done by developing and applying exemplary assessment materials with the potential to improve performance assessment practices associated with demonstration experiments in Physics. The study recommends that the Ministry of Education and Culture and teacher training institutions should promote the training of teachers in developing exemplary assessment materials for their own use in schools. These materials should contain specific guidelines on how to conduct effective assessment practices.