It is widely recognised that rapidly received feedback on assessment tasks has an important role to play in underpinning student learning and encouraging engagement. Within the Open University (OU), increasing use is being made of interactive computer-marked assignments (iCMAs), alongside more conventional tutor-marked assignments (TMAs). OU students are primarily adult distance-learners, studying at home from printed text and various computer-based resources. Most students are allocated to a personal tutor, but opportunities for face-to-face contact are limited. "S104: Exploring Science" is one of several possible starting points for a student wanting to study science with the OU. "S104" is one of a growing number of courses making use of the OpenMark online assessment system. In "S104" and other OU science courses, iCMAs are being used for diagnostic, formative and summative purposes. Once students are studying a course, the main benefits of iCMAs are their provision of rapid feedback and their "pacing" function, helping students to keep up-to-date in their studies. iCMAs can also be used to keep tutors informed of their students' progress.