Enhanced biological phosphorus removal (EBPR) is a widely used, cost-effective technology for eliminating phosphorus from wastewater. Its efficiency depends on the metabolic activity of polyphosphate-accumulating organisms (PAOs) and denitrifying PAOs (DPAOs). During shortcut biological denitrification via the nitrite pathway, nitrite can accumulate in activated sludge and protonate to form free nitrous acid (FNA). Elevated FNA concentrations inhibit PAO activity and destabilise overall EBPR performance. This review therefore summarises the inhibitory effects of FNA on EBPR microorganisms and elucidates the underlying mechanisms. The resulting insights can inform the design and operation of stable, high-efficiency EBPR systems.