How to understand implementation processes is a key problem for implementation science. These processes can be defined as ones in which the strategic intentions of one group of actors are translated into the everyday practices of others. Understanding them calls for us to also understand the ways that these translation processes are motivated and shaped by internal and external mechanisms. Normalization Process Theory identifies and explains the operation of mechanisms that characterize implementation processes. The authors first describe the scope and core components of the theory and consolidate its development and different iterations. Then, they discuss the application of the theory, and consider the ways in which this involves qualitative and quantitative methods that can be used to refine and extend understanding of implementation processes.