The most straightforward approach to estimation of stomatal and boundary layer conductances from thermal imagery depends on the use of wet and/or dry reference surfaces. An alternative approach might be to utilise and active thermography approach based on the thermal dynamics of leaves following changes in incident radiation. The theoretical basis of this approach will be outlined and its limitations evaluated. Potential applications and disadvantages of this approach are outlined and discussed on the basis of a range of laboratory and field experiments on grapevine and other plant species.