Research papers in IoT Application Programming Interfaces (APIs) investigate the design, development, and standardization of programming interfaces that enable seamless interaction between IoT devices, platforms, and applications. These works emphasize how APIs act as the bridge for data exchange, device control, and service integration, making IoT systems more accessible and interoperable. Studies explore various API paradigms such as RESTful APIs, WebSocket-based APIs, GraphQL, and microservice-driven APIs that support lightweight communication, scalability, and real-time responsiveness in resource-constrained environments. Research also highlights the role of standardized APIs defined by frameworks like oneM2M, OCF (Open Connectivity Foundation), and FIWARE in addressing heterogeneity and ensuring cross-platform compatibility. Advanced works analyze how APIs integrate with edge, fog, and cloud infrastructures to support dynamic service orchestration, context awareness, and data-driven decision-making. Security-aware API design is another critical research area, focusing on secure authentication, authorization, encryption, and privacy-preserving access mechanisms such as OAuth 2.0, JSON Web Tokens, and blockchain-enabled API gateways. Application-specific studies demonstrate how IoT APIs power healthcare systems for secure patient data exchange, smart city platforms for integrating urban services, industrial IoT for predictive maintenance, and intelligent transportation for connected vehicle ecosystems. Collectively, this body of literature underscores the importance of APIs as a fundamental enabler of IoT, driving interoperability, innovation, and scalability while paving the way for next-generation connected environments.