Land surface models (LSMs) are pushing towards improved realism owing to an increasing number of observations at the local scale, constantly improving satellite data-sets and the associated methodologies to best exploit such data, improved computing resources, and in response to the user community. As a part of the trend in LSM development, there have been ongoing efforts to improve the representation of the land surface processes in the Interactions between the Surface Biosphere Atmosphere (ISBA) LSM within the EXternalized SURFace (SURFEX) model platform. The Force-Restore approach in ISBA has been replaced in recent years by improved realism with respect to for example, multi-layer explicit physically-based options for sub-surface heat transfer, soil hydrological processes, and the composite snowpack. The representation of vegetation processes in SURFEX has also become much more sophisticated in recent years, including photosynthesis and respiration and biochemical processes. It become clear that the conceptual limits of the composite soil-vegetation scheme within ISBA have been reached and there is a need to explicitly separate the canopy vegetation from the soil surface. In response to this issue, a collaboration began in 2008 between the High-Resolution Limited Area Model (HIRLAM) consortium and Météo-France with the intention to develop an explicit representation of the vegetation in ISBA under the SURFEX platform. A new parameterization has been developed called the ISBA Multi-Energy Budget (MEB) in order to address these issues. ISBA-MEB consists in a fully-implicit numerical coupling between a multi-layer physically-based snowpack model, a variable-layer soil scheme, an explicit litter layer, a bulk vegetation scheme, and the atmosphere. It also includes a feature which permits a coupling transition of the snowpack from the canopy air to the free atmosphere. It shares many of the routines and physics parameterizations with the standard version of ISBA. This paper is the first of two parts: in part one, the ISBA-MEB model equations, numerical schemes and theoretical background are presented. In part two which is a separate companion paper, a local scale evaluation of the new scheme is presented along with a detailed description of the new forest litter scheme.