This article reports on a study which used the APOS (action-process-object-schema) Theory framework and a classification of errors to investigate university students' understanding of the integration concept and its applications. Research was done at the Westville Campus of the University of KwaZulu-Natal in South Africa. The relevant rules for finding antiderivatives, the link between derivatives and antiderivatives, interpreting a definite integral as area under the relevant curve and their context-based applications were taught to undergraduate science students. This paper reports on the analysis of two students' responses to questions on integrals and their applications. The findings of this study suggest that those students had difficulty in applying the rules for integrals and their applications, and this was possibly the result of them not having appropriate mental structures at the process, object and schema levels.