The introduction of marine species to natural environments has been identified as the second greatest threat to biodiversity after habitat destruction. It produces dramatic effects to biological productivity, habitat structure and species composition. The vulnerability of our country to biological invasions has increased due to the boost of human activities, including sea, air and land transportation. The first global assessment of the threat of marine invasive species recognized 329 species with these characteristics; 12 of them are recognized as established non-native species for the Southwestern Caribbean ecoregion, of which eight are harmful species. Similar problems are known to occur in other areas of the region, such as the Greater Antilles, Southern Caribbean, Southern Gulf of Mexico and Florida. However, Colombia does not have yet a marine invasive species inventory, and their potential impacts are poorly understood. To date the only reliable information is limited to four cases, Oreochromis niloticus, Kappaphycus alvarezii Alitta succinea and Pterois volitans (see case studies). Thus, the goal of this book is to present a preliminary illustrated list of alien species identified and recorded in the marine and coastal environments of Colombia, including those species for wich some evidence of their presence exist. This book intents to provide the baseline information about the present status of marine biodiversity in Colombia, which will help managers and policy makers to development plans to face these threats. In addition, it provides a list of species that can be classified as cryptogenic (see definitions). The inclusion of the species is supported in most cases by scientific literature and the study of specimens stored at the Marine Natural History Museum of Colombia (information included in the Marine Biodiversity Information System–SIBM by its acronym in Spanish). The sixteen species identified to date belong to macroorganisms, including the algae Kappaphycus alvarezii, the corals Carijoa riisei and Tubastraea coccinea, the worm Alitta succinea, the bivalves Electroma sp., Corbicula fluminea, Perna perna and Perna viridis, the crustaceans Balanus amphitrite, Penaeus monodon, Charybdis hellerii and Rhithropanopeus harrisii and fishes such as Oreochromis niloticus, Trichogaster pectoralis, Omobranchus punctatus and the recently recorded Pterois volitans. Most of these species are originally from the Indo-Pacific region and the Mediterranean Sea.