iPILPS, isotopes in the Project for the Intercomparison of Land-surface Parameterization Schemes (PILPS), is a new stage in one of the longest running atmospheric/climate model intercomparison projects (MIPs). Its goal is to provide a rigorous framework within which isotopic simulations of the continental surface can be inter-compared, evaluated, validated and/or improved. Operating under the auspices of the Global Energy and Water Cycle Experiment's (GEWEX) Land-Atmosphere System Study, PILPS is an off-line intercomparison and improvement framework offered for land-surface schemes used in weather forecast, climate and Earth System models. This paper describes the motivation for iPILPS, emphasising the energy and water budget difficulties of current land-surface schemes (LSSs) and underlining the need for new tools with which to probe "correctness" of evapotranspiration parameterization. It also explains and describes the likely beneficial outcomes anticipated from including isotopes in land-surface parameterization evaluation. These are the potential to: improve parameterization of surface energy partition by current LSSs; add information leading to refinement of basin-wide water modelling; reduce the range in simulated carbon uptake in the whole biomass and its subcomponents; and fingerprint sources and sinks involved in specific processes allowing evaluation of LSSs and, hopefully, their validation.