Inflammation has been recognized as a core component of obesity-induced metabolic disease.  However, a major question in biology is how is inflammation sensed in circumstances of obesity and nutrient overload? It has become widely accepted that the activation of toll-like receptor 4 (TLR4) in immune cells such as macrophages, links obesity to the development of chronic tissue inflammation and metabolic disease. Specifically, it is thought that long chain saturated free fatty acids, such as palmitate, are agonistic ligands for TLR4 in macrophages, initiating inflammation and consequently promoting the development of obesity related pathologies. Using several different experimental approaches, we provide compelling evidence that long chain saturated fatty acids, such as palmitate are, in fact, not ligands for TLR4 in macrophages. Paradoxically, however, we have demonstrated that bone marrow-derived macrophages from TLR4 knockout or C3H/HeJ (which have a naturally occurring mutation of TLR4, leaving receptor expression intact but preventing the activation of TLR4) mice display a completely suppressed inflammatory response, as measured by phosphorylation of JNK, to palmitate treatment, supporting previous observations (Shi et al. J Clin Invest 116:3015-3025, 2006). We have carefully examined specific signalling pathways and have obtained data which, we believe, reconciles these two seemingly opposing observations. These pathways will be discussed during this presentation.