High-gain free-electron lasers (FELs) have been a great success as the fourth generation light sources. In the last decade, tremendous progress has been made in both theoretical understandings and successful constructions and operations of large-scale FEL facilities all over the world. To generate fully coherent, ultrafast X-rays with high brightness, various novel seeded FEL schemes have been proposed and experimentally demonstrated in recent years. This paper gives an overview of recent advances and prospects for future developments in the seeded high-gain FELs.