Internet of Things (IoT) technology is starting to make an impact in a wide array of applications, including smart cities and industrial environments. Such real-world applications combine computation, communi- cation, sensing, and in some cases, actuation, to monitor and remotely control the environment. Data is at the core of such real-world IoT applications. Analysis, modeling, and reasoning of data are necessary to gain valuable insights. Application developers employ context-aware systems to translate the data into contextual information, which then allows the applications to act cognitively. Context sharing platforms offer a solution to distribute context information to those who may be interested in it, thus enabling context interoperability among different entities. This survey first examines the requirements for sharing context information. It then reviews the relevant literature for context sharing and classifies them based on their requirements and characteristics. Challenges and future directions are presented to encourage the development of context sharing platforms.