Although measurement of serum lactate has an established role in the monitoring of critically ill patients at many hospitals around the world, the practice is by no means universal, reflecting a controversy that still surrounds the topic. In short there remains no consensual answer to the broad question: Should we routinely monitor lactate in the critically ill and if so, when should we measure it? A recently published systematic health technology assessment (HTA) of lactate monitoring in the critically ill was undertaken in an effort to illuminate the controversy. Those responsible for the study formulated eight very concise and specific questions about lactate monitoring and then trawled the literature for studies that provide evidence that would help answer each of the questions posed. A systematic literature search revealed 360 potentially relevant papers. Of these 152 fulfilled the selection criterion of being original research of specific relevance to one of the eight questions. Headline findings of the HTA include the finding that lactate measurement by a range of point-of-care analyzers does not differ in terms of clinically acceptable accuracy and precision from laboratory-based reference methods; lactate measurements are technically reliable. The study finds that lactate should be measured directly; evidence militates against the practice, adopted in some centers, of estimating lactate from other acid-base variables (pH, base excess) or anion gap. The study finds that lactate measurement has both diagnostic and prognostic significance, although there is a lack of clinical trials investigating the clinical value of lactate-directed therapy. It is thus clear that we do not know if therapies that cause normalization of blood lactate improve outcome for patients. The HTA did not reveal any studies that would help answer one of the posed questions: Does lactate measurement increase the confidence of healthcare providers? There have also been very few studies (just two identified here) regarding cost-benefit analysis of lactate monitoring. In conclusion, the authors of this HTA recommend the routine use of lactate monitoring in a critical care setting on the well-evidenced grounds that it is useful for risk stratification of patients suffering many kinds of critical illness. They call for randomized controlled studies on the efficacy of lactate-directed therapy so that a significant gap in our current knowledge surrounding lactate monitoring, highlighted by this study, can be closed. An accompanying editorial states that at least two such studies are now underway. Overall the paper is a reliable synthesis of much of what we do, and do not know about lactate monitoring of the critically ill.