Gaining some momentum in the early 18th century, accelerating in the 19th century, and culminating in the second half of the 20th century, a sharp increase in the number of non-native animal and plant species introduced to foreign ecosystems, terrestrial and aquatic alike, occurred. In marine systems, they appeared in the pelagic as well as in near-shore zones. Like an infection, this process rapidly spread globally. Currently, the ?spontaneous? appearance of invasive, non-native species in various taxonomic groups, ranging from algae and protozoa to fishes, and their naturalization to new biotopes has become like an epidemic to coastal regions of the World Ocean, inland seas, and to brackish and freshwater systems as well. This book provides a comprehensive synthesis of all aspects regarding the non-native species in the Ponto-Caspian seas, including their introduction, naturalization, geographical distribution and consequences for native biota with special attention to gelatinous species. Highlighting important research findings, this book provides an overview of the invasion process from transportation patterns and reasons for establishment success to ecological impacts and role of abiotic factors and climate change. Increasing awareness of the problems associated with invasions of non-native species in Ponto-Caspian seas has led to rapid growth in research on this topic. This book provides updated summary of this fast-growing field of research, and is an essential text for established researchers, undergraduate and graduate students.