Lipids and lipid metabolism have well-documented regulatory effects on inflammatory processes. Recent work has highlighted the role of the peroxisome proliferator-activated receptors (PPARs) — a subset of the nuclear-hormone-receptor superfamily that are activated by various lipid species — in regulating inflammatory responses. Here, we describe how the PPARs, through their interactions with transcription factors and other cell-signalling systems, have important regulatory roles in innate and adaptive immunity.