Cell-free (CF) integrated sensing and communication (ISAC) merges the CF architecture with ISAC functionalities. CF-ISAC leverages distributed access points, removes cell boundaries, and enhances coverage, spectral efficiency, and reliability. It also improves energy efficiency, enabling robust multi-user communication, distributed multi-static sensing, and seamless resource optimization. A comprehensive survey on CF-ISAC has been lacking. This monograph addresses that gap by covering the foundational principles, cooperative transmission, radar cross-section, target parameter estimation, ISAC integration levels, sensing metrics, and key applications. It also explores the advantages of multi-static sensing. Performance analysis, resource allocation, security, and user/ target-centric designs are discussed. Finally, synchronization, multi-target detection, interference management, and fron-thaul limitations are discussed. Advanced antenna technologies, network-assisted systems, near-field CF-ISAC, cross-technology integration, and machine learning approaches are presented. Diluka Galappaththige and Chintha Tellambura (2025), “Cell-free Integrated Sensing and Communication”, Foundations and Trends® in Networking: Vol. 16, No. 1-2, pp 1-170. DOI: 10.1561/1300000079. ©2025 D. Galappaththige and C. Tellambura