The dramatic growth in demand for wireless services has fueled a severe spectrum shortage, especially in the overcrowded unlicensed bands. The regulatory approach for meeting this galloping demand is to allow the coexistence of competing wireless technologies, cellular, Wi-Fi, radar, TV, emergency communications, and others. This shared spectrum paradigm poses novel challenges for the secure, efficient, and fair resource allocation. Many of these challenges stem from the heterogeneity of the coexisting systems, their scale, and the lack of explicit coordination mechanisms. Whereas some recent efforts have tried to address the coexistence of specific technologies, a comprehensive and general approach to securely and efficiently coordinate spectrum access for heterogeneous systems remains elusive. By facilitating the fair and secure coexistence of heterogeneous wireless systems, this project directly addresses the spectrum scarcity challenge. The outcomes of this project will benefit many industry verticals such as transportation, manufacturing, agriculture, critical national infrastructure, telecommunications, and others. Moreover, the expected results will advance knowledge in a number of scientific fields including security, privacy, and communication theory. Training opportunities for future wireless and security experts are also provided. This project focuses on developing a novel coexistence framework for coordinating, monitoring, evaluating, and adapting spectrum access in a secure, efficient, and privacy-preserving manner. A general coexistence model in which spectrum can be horizontally and/or vertically shared in time, frequency, and space is considered. For this model, new fairness mechanisms that go beyond airtime sharing and account for unfairness due to spatial multiplexing are being investigated. These mechanisms dynamically allocate resources, not only in frequency and time but also spatially, yielding a fairer coexistence. Moreover, unfairness scenarios due to selfishly ignoring the coexistence etiquette are being modeled and assessed. The critical challenge is to detect and mitigate selfish misbehavior at the device and system level without explicit signaling. To overcome this challenge, the project explores implicit physical layer sensing and monitoring mechanisms that infer various operational attributes and safeguard spectrum access. Finally, the interplay between privacy, interference management, and efficiency in vertical spectrum sharing is investigated. The goal here is to protect the location privacy of assets deployed by legacy operators. Adaptive location obfuscation methods are being developed for achieving a desired degree of privacy while maximizing the spectrum efficiency.