Cooperative ISAC networks integrate sensing and communication, offering enhanced performance in wireless networks. This paper explores the opportunities and challenges of network-level ISAC, focusing on new metrics, optimization of degrees of freedom (DoF), cooperation regimes, and S&C tradeoffs. It discusses cooperative architectures at task, data, and signal levels, and investigates the interplay between S&C at the network level. Key challenges include inter-cell interference, limited coverage, and resource constraints. The paper highlights the need for a cooperation framework that balances S&C performance gains with control signaling costs. It also addresses new technical challenges in wireless resource allocation and user/target scheduling. Network-level ISAC offers new opportunities for improved S&C performance by overcoming the limitations of conventional link/system level ISAC. It discusses scenarios such as space-air-ground ISAC networks, multi-modal sensing information transmission and fusion, and vehicular ISAC networks. The paper also identifies challenges such as network synchronization requirements, limited backhaul constraints, and security and privacy in networked ISAC. The paper categorizes ISAC network cooperation into four groups: coordinated cell association, collaborative data fusion, cooperative interference management, and joint cooperative S&C arrangements. It discusses the benefits, requirements, and challenges of each cooperation level. The paper also explores the synergies between network sensing and communication, highlighting the mutual benefits of networked S&C. The paper presents a case study on interference management and cooperation schemes, demonstrating the efficiency of cooperative schemes in ISAC networks. It also outlines future research directions, including smart propagation engineering, semantically aware ISAC networks, and self-adaptive AI in ISAC networks. The paper concludes that network-level ISAC has the potential to improve S&C performance by leveraging network-level DoF, and highlights the essential challenges and opportunities in ISAC networks.Cooperative ISAC networks integrate sensing and communication, offering enhanced performance in wireless networks. This paper explores the opportunities and challenges of network-level ISAC, focusing on new metrics, optimization of degrees of freedom (DoF), cooperation regimes, and S&C tradeoffs. It discusses cooperative architectures at task, data, and signal levels, and investigates the interplay between S&C at the network level. Key challenges include inter-cell interference, limited coverage, and resource constraints. The paper highlights the need for a cooperation framework that balances S&C performance gains with control signaling costs. It also addresses new technical challenges in wireless resource allocation and user/target scheduling. Network-level ISAC offers new opportunities for improved S&C performance by overcoming the limitations of conventional link/system level ISAC. It discusses scenarios such as space-air-ground ISAC networks, multi-modal sensing information transmission and fusion, and vehicular ISAC networks. The paper also identifies challenges such as network synchronization requirements, limited backhaul constraints, and security and privacy in networked ISAC. The paper categorizes ISAC network cooperation into four groups: coordinated cell association, collaborative data fusion, cooperative interference management, and joint cooperative S&C arrangements. It discusses the benefits, requirements, and challenges of each cooperation level. The paper also explores the synergies between network sensing and communication, highlighting the mutual benefits of networked S&C. The paper presents a case study on interference management and cooperation schemes, demonstrating the efficiency of cooperative schemes in ISAC networks. It also outlines future research directions, including smart propagation engineering, semantically aware ISAC networks, and self-adaptive AI in ISAC networks. The paper concludes that network-level ISAC has the potential to improve S&C performance by leveraging network-level DoF, and highlights the essential challenges and opportunities in ISAC networks.