Abstract: Grid-forming (GFM) control for inverter-based resources (IBRs) is inevitable for accommodating high-penetration renewables in power systems. It allows IBRs to provide voltage and frequency support by mimicking the characteristics of synchronous generators. Protection is the first defense line that can ensure the safe and stable operation of the power system. Its performance depends on the fault characteristics, which are highly related to the physical limits and control strategies of inverters in the power system dominated by GFM IBRs. First, this paper outlines the control strategies and characteristics of typical GFM inverters during normal operation and fault conditions. Then, it summarizes the achievement of equivalent models and fault characteristics of the sources. Finally, it sums up the research status of protection for the GFM IBR integrated system and preliminarily evaluates its impact on the protection and future challenges.