The blue crab, Callinectes sapidus Rathbun, 1896, is a portunid crab native to the western Atlantic coasts. The species has been introduced in European waters at the start of the 20th century, and at present is widespread in the Mediterranean Sea. Noticeably, the occurrence of established populations of the species has been repeatedly reported in the Adriatic and Ionian Sea; however, reports have been generally qualitative. This book presents an updated review of the occurrence of Callinectes sapidus along the coasts of the Salento peninsula (South East Italy), together with quantitative estimations of the abundance patterns of the species in two coastal habitats located on the Adriatic and Ionian Sea. In addition, an assessment of the trophic position of Callinectes sapidus in summer months is provided using carbon and nitrogen stable isotope analysis. The book provides novel information on the occurrence and potential impact of this invasive species in Italian coastal habitats, highlighting current knowledge lacunae and identifying future research lines to be addressed in order to understand the species’ expansion in the Mediterranean Sea and its effects on autochthonous biota.