The domain of Internet of Things (IoT) is rapidly expanding beyond research, and becoming a major industrial market with such stakeholders as major manufacturers of chips and connected entities (i.e., things), and fast-growing operators of wide-area networks. Importantly, this emerging domain is driven by applications that leverage an IoT infrastructure to provide users with innovative, high-value services. IoT infrastructures range from small scale (e.g., homes and personal health) to large scale (e.g., cities and transportation systems). In this paper, we argue that there is a continuum between orchestrating connected entities in the small and in the large. We propose a unified approach to application development, which covers this spectrum. To do so, we examine the requirements for orchestrating connected entities and address them with domain-specific design concepts. We then show how to map these design concepts into dedicated programming patterns and runtime mechanisms. Our work revolves around domain-specific concepts and notations, integrated into a tool-based design methodology and dedicated to develop IoT applications. We have applied our work across a spectrum of infrastructure sizes, ranging from an automated pilot in avionics, to an assisted living platform for the home of seniors, to a parking management system in a smart city.