The netted radar systems are known for being able to provide higher target detection probabilities, more accurate measurements, and better multi-target tracking performance than the monostatic/bistatic radar systems by exploiting information fusion processing. Simple radar networks made of multiple independent monostatic radars with decentralized processing have been around for several decades. In contrast, sophisticated radar networks with centralized data fusion only emerged during the last few years thanks to the development in high-speed digital processing and precise time/phase synchronization. Although the advanced radar networks offer the potential ability to detect stealth targets and low probability of intercept radar signal, there are many technical challenges to be solved before they could evolve from concepts in the research paper to implementable systems in the real-world. By presenting the various target detection and parameter estimation approaches adopted by the distributed MIMO radar, the passive radar network, the hybrid active-passive radar network, and the MIMO synthetic aperture radar (SAR), we lay out the foundation for the future research to be conducted to solve the key issues limiting the practical performance of these advanced radar networks.