Many indoor localization algorithms have been proposed to enable location-based applications in indoor environments. However, these systems are monolithic and not componentbased. We present BearLoc, a distributed modular framework for indoor localization systems that provides (1) natural development abstractions for sensor, algorithm, and application components, and (2) easy and flexible component composition. We demonstrate the merits of BearLoc with an example use case. Our evaluation shows we can reduce developer lines of code by 60% while introducing acceptable network delay overhead. Categories and Subject Descriptors D.2.11 [Software Engineering]: Software Architectures— Patterns (e.g., client/server, pipeline, blackboard), Domainspecific architectures; C.2.4 [Computer-Communication Networks]: Distributed Systems—Distributed applications; D.2.12 [Software Engineering]: Interoperability—Distributed objects Keywords Localization; Framework; Composability