The present contribution reviews the operando spectroscopy programme in the area of homogeneous catalysis that exists at the Institute of Chemical and Engineering Sciences (ICES). This review first considers the importance of experimental design and then addresses a number of signal processing issues. Band-target entropy minimization (BTEM), an algorithm designed for pure component spectral estimation, is the most important signal processing tool discussed. A homogeneous catalytic example using BTEM analysis, as well as other quantitative and kinetic analyses is shown. In addition to spectroscopic measurements, other in situ sensor measurements and their analyses are also discussed. The determination of individual physico-chemical properties of the constituents present allows insight into solute-solute interactions, etc. This review is based on a plenary lecture presented at Operando III. (C) 2009 Elsevier B.V. All rights reserved.