The anatomy of water, water as a substance, water as a medium, the principles of the hydrologic cycle, the economics of water, and challenges are all covered in the first chapter of this book. The horizon of the tropical world, the environment, particularly the tropical environment, aquatic biome, tropical aquatic bionetwork, concept of biosphere, and tropical limnology are all covered in the second chapter. The third chapter covers the following topics: the origins of lakes, general lake classification, tropical lakes, lake morphometry, morpho-edaphic index, trophic status index of lakes, wetlands, and mangroves in tropical regions. The lotic environment is the main topic of the fourth chapter, which also covers the idea of stream order, the differences between rivers and streams, the river continuum, physical, chemical, and biological characteristics, and adaptations of fish found in hill streams. Chapter five covers the prokaryota, cyanobacteria, freshwater biota, and water-adapted organisms. The sixth chapter focuses on the algal communities Xanthophyceae, Euglenophyceae, Bacillariophyceae, Chrysophyceae, Phaeophyceae (brown algae), and Chlorophyceae. The seventh and last chapter covers the following topics: Protozoa, Porifera, Rotifera, Coelenterata, Annelida, Arthropoda, Crustacea, Aquatic Insects, Mollusca, Echinodermata, and Brachiopodaa.