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We have constructed a grid of about 10,000 spherically symmetric andplane-parallel models with the MARCS program, and make it available for publicuse. Parameter ranges are: Teff=2500 to 8000 K, log g =log(GM/R2)= -1 to 5(cgs) with various masses and radii, [Me/H]=-5 to +1, with [Alpha/Fe] = 0.0 and0.4 and different choices of C and N abundances to also represent stars oftypes R, S and N, and with microturbulence parameters from 1 to 5 km/s. We alsolist fluxes in approximately 108,000 wavelength points. Underlying assumptionsin addition to 1D stratification include hydrostatic equilibrium, MLTconvection and LTE. A number of general properties of the models are discussed,in relation to the effects of changing blanketing and sphericity.  Models are compared with other available grids and excellent agreement isfound with plane-parallel models of Castelli and Kurucz within the overlappingparameter range. Although there are departures from the spherically symmetricNextGen models, the agreement with more recent PHOENIX models is gratifying.The models of the grid show regularities, but some interesting departures fromgeneral patterns occur for the coolest models due to the molecular opacities.We have tested rules of thumb concerning effects of blanketing and sphericityand found them to often be astonishingly accurate. Some interesting newphenomena have been discovered, such as the intricate coupling betweenblanketing and sphericity, and the strong effects of carbon enhancement onmetal-poor models. We give further details of models and comparisons withobservations in subsequent papers.

A grid of MARCS model atmospheres for late-type stars I. Methods and general properties