Desalination simply means the process of obtaining potable water form saline water. The primary desalination methods are multi-stage flash distillation and reverse osmosis. The first step in any improvement is diagnostics, and, the most powerful diagnostics tool in thermodynamics is the second-law analysis. The starting point of any second-law analysis is the study of the process under reversible conditions. Reversible processes are relatively easy to analyze since they deal with point functions, and the reversible work is independent of the process path followed. The reversible operation represents the best case scenario for any process or device, and it can be used as a basis of comparison for actual processes or devices, and to evaluate second-law efficiencies. This then can serve as a starting point for the improvement of actual processes. In this work the overall desalination process is analyzed, and the minimum work input requirements are determined under various conditions together with case studies. These values can then be used to assess the thermodynamic performance of actual desalination plants under specified operating conditions.