Computer simulations of two-dimensional (2D) fluids are discussed in connection with their use in determining the isotherms and thermodynamic properties of monolayers adsorbed on model surfaces. The use of powerful algorithms such as the Grand Canonical Monte Carlo (GCMC) to simulate a greatly expanded range of adsorption systems is then discussed in connection with computations of multilayer adsorption, adsorption on heterogeneous surfaces and in pores. (C) 2002 Published by Elsevier Science B.V.