In recent years various papers have been published on the practical use of infrared thermometry and thermal imaging to estimate leaf stomatal resistance [(amongst others: Jones, H.G., 1999a,b; Leinonen et al., 2006). Use of infrared thermometry for estimation of stomatal conductance as a possible aid to irrigation scheduling. Agricultural and Forest Meteorology 95, 139-149; Jones, H.G., 1999b. Use of thermography for quantitative studies of spatial and temporal variation of stomatal conductance over leaf surfaces. Plant Cell and Environment 22, 1043-1055; Leinonen, L, Grant, O.M., Tagliavia, C.P.P., Chaves, M.M., Jones, H.G., 2006. Estimating stomatal conductance with thermal imagery. Plant, Cell & Environment 29, 1508-1518]. These papers are based on the physical relationships linking the stomatal resistance of a leaf to its own temperature and to the temperatures of reference leaves (dry and wet). These relationships differ depending on the type of leaf (amphistomatous or with stomata on only one side) and on the way the reference leaves are wetted (on either both sides or on one side only). Because of ambiguous assumptions on the wetting process and small inaccuracies in the calculation of some resistances, these basic relationships are not clear. The present paper aims to clarify and synthesize the appropriate equations for each case.