Conceptual models are graphical representations of the relevant features of a system domain. They play a significant role in the early detection and correction of information systems development errors and help analysts, developers, and end users to better understand the domain. However, despite conceptual modelling being an active research area in information systems, much understanding of conceptual modelling practice remains outstanding. This thesis addresses such aspect: while practitioners

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... typically use multiple, and different, types of models in system analysis and design tasks, no intense scrutiny of how and why they use model combinations has been undertaken. The objective of this research is to provide an in-depth investigation into the use of model combinations during information systems analysis and design. A multi-method research design was used to achieve this objective. The contribution of this thesis is built on three streams of studies: a systematic literature review, exploratory interviews, and a lab experiment for theory testing. First, a systematic literature review was conducted to analyse the extent to which research over the past two decades has identified research opportunities and provided answers to Wand and Weber's (2002) question regarding "How can we model the world to better facilitate our developing, implementing, using, and maintaining more valuable information systems?". The next two studies focussed on the theory of faithful use of conceptual model combinations. This theory was developed to predict and explain the use of multiple models in combination; however, this theory has not yet been tested. In the second study, semistructured interviews were conducted to explore and determine the circumstances that lead information system (IS) professionals to use multiple models. In the third study, lab experiments were designed and executed to test the hypotheses derived from the propositions of the theory of faithful use of conceptual model combinations. The results led to a refined and improved theory that provides explanation and in-depth understanding of how and why practitioners use model combinations. iv