Linear techniques probe all the molecular systems present in the expansion, based on their photon absorption or scattering cross sections. They can usually identify size (e.g. hydration number) by variation of the expansion conditions and by following the signal evolution, at least for 1:1 complexes if those do not spectrally overlap with other cluster compositions. Conformations can only be distinguished if they differ in their spectral fingerprint, sometimes with the help of isotope substitution. There is a less liberal molecular size limit for linear techniques due to the decreasing vapour pressure and the limited sensitivity. Action techniques may in turn suffer from unwanted competing processes such as fragmentation and may still be problematic for many non-aromatic systems and even some aromatic systems with fast processes in the electronically excited state . The best is always a combination of both techniques, such as in the case of POH from the training set. This is, however, unrealistic on the time scale of this challenge and for the choice of molecules. It is anticipated that one member of the test set (PCD) will only be characterised by an action technique (IR/UV double resonance), whereas the others will be mostly accessed by direct infrared absorption. For the latter, a recent instrumental development is very helpful for monohydrates, because its gas recycling allows for the use of expensive isotopologues, solutes and carrier gases. In some cases, where the water vibration of interest has a low infrared intensity or may be distributed over several OH stretching modes in the complex, linear Raman spectroscopy can provide the required information.