Blood lactate levels are a simple and inexpensive parameter that is increasingly available to practitioners in both emergency medicine and general practice. But what does it really tell us? The earliest studies in people dating from the 1970s were the first to advocate lactate as a prognostic indicator with one of the landmark studies showing that as admission lactate concentration increased from 2.1 to 8.0 mmol/L survival decreased from 90% to 10%. When treating emergency patients with distressed owners, there is something very attractive about the idea of measuring a simple parameter on admission that could give us a better sense of prognosis than our clinical opinion alone... But does the data support that? In this session we will briefly review lactate physiology and then focus on the small animal veterinary evidence base – how has it evolved, what do we now know and where are the gaps in knowledge and potential misinterpretations that could affect our use of this clinical tool.