We start with the recent developments and reasons why the meromictic lakes should get more attention in limnological literature. Thereafter, we define monimolimnion , mixolimnion and some other terms related to meromixis. The definition of Hutchinson for meromictic lakes holds for more than 60 years while some modification is required. We differentiate between a meromictic lake and a non-meromictic ones, despite the fact that for deep waters the patterns of recirculation in non-meromictic lakes may be similar to meromixis. We also refer briefly to the classification schemes of meromixis and list processes involved in sustaining stratification. Regions on the Globe that produce salinity gradients in surface waters; mining areas, where geochemistry is more actively operative; and areas where waters of different compositions meet may promote the development of such conditions in lakes. Meromixis is a much more dynamic process than it is often understood. If monimolimnion gets eroded, the meromixis can terminate, while changes in the hydrology can turn a lake meromictic. Finally, we briefly introduce here the structure of this book, which has three parts. Part I focuses on physical (Chap. 2), chemical (Chap. 3) and biological (Chap. 4) properties of meromictic lakes. Part II presents eight case studies in separate chapters including the regional studies on different meromictic lakes in five continents. Part III (Chap. 13) is based mainly on the brief conclusion summaries derived from the preceding 12 chapters with editors’ remarks. Lastly, this book is an attempt to update our available knowhow and expertise on the existing state-of-the-art information on meromictic lakes and the literature references that can form the basis for future studies on meromictic lakes.