Flame retardant polymeric nanocomposites have received extensive attention over last decades because of their exceptional flame retardancy and mechanical robustness. Despite great advances, there remains an urgent lack of a comprehensive review on the design, performances, applications and challenges of these polymeric nanocomposites. This review focuses on the state-of-the-art design and performances of polymeric nanocomposites. Substantial emphasis is placed on the combination effect between nanomaterials with variable dimensions and scales, and/or conventional phosphorus-, nitrogen- and/or silicon- containing flame retardants. The combination of nanomaterials with different geometries and conventional flame retardants via either chemical or physical methods has enabled tunable flame retardancy and mechanical performances for the resultant polymeric nanocomposites. Flame retardant mechanisms of polymer nanocomposites are also discussed in detail. Although the multiple combination represents an innovative and effective strategy to create high-performance flame retardant polymeric nanocomposites, many key challenges remain to be addressed. In response to these challenges, four fundamental design principles are proposed for creating flame retardant polymeric nanocomposites. Finally, some potential applications of these advanced flame retardant polymeric nanocomposites are also discussed.