The triggering receptor expressed on myeloid cells (TREM)-1 is a recently identified molecule involved in the inflammatory response. It belongs to the immunoglobulin superfamily and is expressed on the surface of neutrophils, mature monocytes, and macrophages. The engagement of TREM-1 synergizes with the Toll-like receptors signaling pathway in amplifying the inflammatory response mediated by several microbial components. The expression of the membrane-bound TREM-1 is strongly upregulated on monocytes during sepsis. Besides its membranous form, a soluble counterpart of TREM-1 exists that is specifically released during several infectious processes. The measurement of that soluble form in biological fluids may be useful as a diagnostic tool, especially during severe sepsis and pneumonia. Moreover, the evolutionary pattern of TREM-1 may be interesting during the follow-up of septic patients.