In this chapter we present the most important simplified free energy methods in use today and the main biological problems that have motivated their development. The area in which these methods are perhaps the most valuable is the study of molecular recognition between biological molecules, such as an enzyme and a substrate or inhibitor. Noncovalent association between biomolecules is a key element of the biochemistry and information flow in living systems. Many competing effects can contribute to receptor-ligand binding [1]: changes in rotational, translational, conformational, and vibrational entropy of the partners, entropy changes associated with solvent ordering around hydrophobic or charged groups, solute conformational strain, changes in electrostatic and van der Waals interactions within and between the partners and the solvent, counterion reorganization. Experimental studies often combine structure determination methods with point mutagenesis and thermodynamic measurements to obtain information on the binding [2]. However, there are considerable difficulties in the experimental analysis of longer-range electrostatic contributions, the cooperativity between amino acid residues of a protein, or disordered solvent, for example. Such effects can be determined using rigorous free energy simulations, described in the earlier chapters of this book. They can also be incorporated, at different levels of accuracy, into simplified free energy methods.The basic principle of a receptor-ligand binding analysis by free energy simulations is explained in Fig. 12.1 [3]. Most applications focus on binding free energy differences between a series of ligands or protein mutants. For a review of the relevant statistical thermodynamics see [1]. Like experiments, the rigorous free energy simulation method requires a reversible (or near-reversible [4,5]) path between the initial and final states. The Helmholtz free energy change along the horizontal legs in Fig