Unsaturated polyester resins (UPRs) are versatile thermosetting materials available in a wide range from general-purpose (e.g., construction) to advanced (e.g., aerospace) materials. UPRs are best known for their promising mechanical and solvent-resistive properties, but they are highly flammable necessitating reinforcing with flame retardants (FRs). In this paper, composites of UPRs are ranked as of Poor, Good or Excellent, based on a comprehensive analysis made on cone calorimetry datasets applied by using Flame Retardancy Index (FRI). Moreover, mechanisms behind flame retardancy performance of UPRs are discussed. FRs used in reinforcing UPRs are generally divided into phosphorus (P), non-phosphorus (NP) and hybrid (any combination of P and/or NP) classes. Besides FRI, available UL-94 and limiting oxygen index (LOI) data were used to investigate whether or not they could correlate with the FRI-based outcomes. However, exploring an explicit correlation was not possible because of discrepancy or lack of data. The performance of FRs incorporated into UPRs for making them flame-retardant was mechanistically discussed based on analyses conducted on char residue. The charring effect along with cross-linking and catalytic effects supporting free radical scavenger mechanism were found to be the main reasons for flame retardancy enhancement of UPRs. Mechanistic insights into flame retardancy action were also discussed briefly in terms of action of FRs in gas and/or condensed phases.