Modularity is the single most important concept related to software and digitalization, and one that every manager and many experts should understand. This chapter elaborates modularity as a central response to complexity that relates to the design, production, and operation of technological systems. Digital technologies increase modularity by facilitating the creation of clear interfaces between sub-systems and processes. This, in turn, allows complex systems to be split into autonomous parts that can be developed and installed independently. Modularity allows companies to scale up their activities more quickly and cheaply, while also facilitating innovation both within modules, by eliminating interdependencies that hamper development efforts, and at the architectural level, through the creative recombination of pre-existing modules.