Successful implementation of health informatics systems depends not only on efficient performance of intended tasks, but also integration into existing working relationships and environments. Implementation is an understudied area in health informatics research, and relevant empirical evidence is often absent from strategic decision making. Implementation theories such as Normalization Process Theory (NPT) can help address this gap by providing explanations for relevant phenomena, proposing important research questions, and framing collection and analysis of data. NPT identifies, characterizes, and explains mechanisms that have been empirically demonstrated to affect implementation processes and outcomes. These explanations are generalizable and facilitate comparative investigations. The first section of this chapter introduces the four main constructs of NPT (coherence, cognitive participation, collective action, and reflexive monitoring) and their constituent components. Each component is discussed with reference to a real-world example, and relationships between the four constructs are explored. The second section explores how NPT has been applied in both prospective planning of interventions and their evaluation, as well as retrospective exploration of factors promoting or inhibiting successful implementation. We examine two examples from published literature: firstly, prospective planning of an evaluation study on implementation of a digital health intervention for Type-2 diabetes; and secondly an evaluation of implementation of a new electronic preoperative information system within a surgical pre-assessment clinic. The chapter concludes with reflections on some limitations of NPT as a theoretical framework.