The development of diagnostic methods suitable for the monitoring of practical flames is an important objective, which is receiving a growing attention and significant research efforts. This is motivated by the need to achieve a more precise description of the process and, ultimately, implement efficient and reliable control and optimisation methods as a key step towards the development of more efficient, flexible, reliable and clean combustion systems. Many and interesting attempts have been proposed, involving widely different approaches in terms of the instrumentation utilized and the concepts proposed to convert sensorial information into various meaningful parameters. This article intends to review the techniques proposed in the literature for the monitoring of flames, either applied to or conceived for the monitoring of practical combustion equipment. It has been divided into four sections, dealing respectively with optical sensors, imaging techniques, pressure transducers and probing methods. A detailed analysis of the works published reveals that probably the main challenge in this field is the definition of the most representative flame signals and of their subsequent processing to derive the meaningful information required to diagnose the state of a flame or to drive a controller in an effective and safe manner. This together with the wide range of diagnostic needs and restrictions imposed by the different combustion situations probably explains the notable heterogeneity observed among the works published. In spite of the great efforts devoted, the techniques proposed for the advanced monitoring of practical flames are still at a development stage. However, significant advances in this field are expected in a near future, fostered by the urgent demands from the combustion industry and facilitated by the continuous progress in sensor technology, signal processing techniques and, not the least, in the understanding of combustion processes. (C) 2009 Elsevier Ltd. All rights reserved.