Invertebrates are common in marine and freshwater ecosystems and key elements in processes such as trophic dynamics and nutrient recycling. At the present time we have a limited knowledge of their diversity and how they have evolved over time. A key element of study in this book, are the current efforts to produce revised classificatory systems which include modern approaches and an update of the current taxonomical system. Another topic discussed in great detail is the relation of invertebrates and their contribution to biodiversity in terms of unique species per habitat. Invertebrates: Classification, Evolution and Biodiversity represents an effort to show the relevance of this unique group of living species and the current efforts to better understand them. This book presents a well balanced set of chapters in which a detailed review of the classification of poorly known groups (such as freshwater arthropods, oligochaetes, and marine ascidians) are studied. Another set of chapters are devoted to understanding the associated biodiversity of invertebrates in marine ecosystems and how they respond to environmental variables. Biogeographical comparisons and evolutionary fractal analysis close the contributions and provide a high standard of knowledge towards the goal of this book. This book also discusses the role of invertebrates in coastal systems and provides key investigation for undergraduate and graduate students and researchers and professional researchers who are interested in ecology and evolution.