The paper presents a simple but realistic biosphere model (SiB) designed for use in general circulation models (GCMs) to calculate the transfer of energy, mass, and momentum between the atmosphere and the vegetated surface of the Earth. The model represents vegetation in each terrestrial grid area as two distinct layers: an upper layer for perennial canopy vegetation and a lower layer for annual ground cover vegetation. The model includes seven prognostic physical-state variables: two temperatures (one for the canopy and one for the ground cover and soil surface), two interception water stores (one for the canopy and one for the ground cover), and three soil moisture stores. The model accounts for radiation absorption, evapotranspiration, momentum transfer, soil moisture availability, and insulation effects. The governing equations for these processes are derived, and the model's structure, including atmospheric boundary conditions, morphological and physiological parameters, and prognostic variables, is described. The paper also discusses the radiation fluxes, aerodynamic resistances, and the calculation of momentum fluxes between the atmosphere and the surface. The SiB model aims to improve the realism of fluxes of sensible heat, latent heat, and momentum over continents compared to existing formulations.The paper presents a simple but realistic biosphere model (SiB) designed for use in general circulation models (GCMs) to calculate the transfer of energy, mass, and momentum between the atmosphere and the vegetated surface of the Earth. The model represents vegetation in each terrestrial grid area as two distinct layers: an upper layer for perennial canopy vegetation and a lower layer for annual ground cover vegetation. The model includes seven prognostic physical-state variables: two temperatures (one for the canopy and one for the ground cover and soil surface), two interception water stores (one for the canopy and one for the ground cover), and three soil moisture stores. The model accounts for radiation absorption, evapotranspiration, momentum transfer, soil moisture availability, and insulation effects. The governing equations for these processes are derived, and the model's structure, including atmospheric boundary conditions, morphological and physiological parameters, and prognostic variables, is described. The paper also discusses the radiation fluxes, aerodynamic resistances, and the calculation of momentum fluxes between the atmosphere and the surface. The SiB model aims to improve the realism of fluxes of sensible heat, latent heat, and momentum over continents compared to existing formulations.