Understanding how new clinical techniques, technologies and other complex interventions become routinely embedded in practice is important to researchers, clinicians, health service managers and policy-makers. Normalization Process Theory (NPT) provides a conceptual framework for understanding and evaluating the processes (implementation) by which new health technologies and other complex interventions are routinely operationalized in everyday work (embedding), and sustained in practice (integration). It has practical value - helping us understand how new ways of thinking, acting and organizing become embedded in healthcare systems. And it offers a conceptual map for the process evaluation of complex interventions and for the organization of implementation processes.

NPT proposes that implementation and integration should be understood by reference to the work that people do. To investigate this we need to consider four further questions. In relation to any complex intervention, we need to ask:

• What is the work? In particular, we are interested in work that defines and organizes the components of a complex intervention, and its relationship with the contexts in which it is set.
• Who gets to do the work? In particular, we are interested in work that defines and organizes the people involved in a complex intervention
• How does the work get done? In particular, we are interested in work that defines and organizes the enacting of a complex intervention.
• How is the work understood? In particular, we are interested in work that defines and organizes assessment of the outcomes of a complex intervention

We recognise that how a complex intervention is diffused and adopted by an organization is an important source of its coherence, and that peoples' attitudes and intentions are important elements of their cognitive participation in a complex intervention, but NPT focuses on their collective and individual work - sometimes very creative, sometimes very reluctant - to achieve a set of collective goals. This means that NPT focuses mainly on the observable collective action of enacting a complex intervention in practice. These are important questions in any intervention design or process evaluation. Importantly, like the rest of NPT, they are drawn from empirical studies of real world complex interventions, and they are highly flexible in their application.

NPT is a sociological toolkit that we can use to understand the dynamics of implementing, embedding, and integrating some new technology or complex intervention. It helps us disassemble the human processes that are at work when we encounter a new set of practices - whether they're bundled around a large randomized controlled trial across many sites, or in a falls prevention program on a single hospital ward. NPT has a robust theoretical basis - and in this section we want to do two things. Explain what a theory is and describe the work it can do, and describe the formal propositions on which NPT is based.