Introduction 351

Part I. A succession of concepts 352

1. Cooperativity 352

2. Cosolvent interaction 352

3. Linearity 354

4. Solvent exchange 354

5. Excluded volume 356

6. Summation 357

Part II

1. The Kirkwood–Buff approach 357

Acknowledgments 360

References 360

In Part I the history of progress in the stabilization and destabilization of protein conformations by means of cosolvents is outlined in terms of distinct conceptual steps. In Part II it is shown that a straightforward application of the Kirkwood–Buff theory of solutions leads to formulas for the preferential interaction and the free energy of unfolding, which confirm and generalize the results of Part I.