It has been argued that design and technology can be used as a vehicle for teaching science and vice versa. In this paper, we report an investigation into Grade 6 students' understandings of concepts embodied in the Systems strand of a technology syllabus as they grappled with a unit of work in technology for the first time. The unit of work involved students in the design and construction of simple systems, followed by experiences with more complex systems. Qualitative research methods were used to investigate the activities of the students as they engaged in the design and technology learning experiences for eight weeks. Data sources included student and teacher interviews, video and audio recordings of whole class and small group interactions, students' notes, drawings and diagrams, and researchers' field notes and reflections. Changes in students' ability to identify the individual components and analyse how the components worked together in systems, and evidence of students' use of scientific understandings for explanations, were noted as the unit of work progressed. An improvement was observed in students' abilities to describe relationships between inputs, processes and outputs, and outline sequences of cause and effect. Recommendations are proposed for enhancing the value of both design and technology and science if both subjects are taught together as part of an integrated program in primary schools.