This volume containe a selected papers related with the more important aspects of the middle-sized pelagic fishes. These fishes are characterized by theirs fast growth, broader food spectrum, high horitzontal and vertical mobility. These traits allow these fishes to thrive under environmental and fishery conditions unfavourable to small pelagics and to develop large shoals and attain high biomass levels capable of supporting high levels of fishing activity. The overal picture will be broken down into seven separate topics: a) a review of the present state of biological knowledge; b) fish behaviour, a basic aspect of the biology and ecology of these species, with special attention for juvenile and adult stages; c) ecological structure: expansion and replacement; d) repercussions of environmental changes; e) fluctuations in abundance, highly characteristic of these species; f) anthropogenic impact: fishing activity on middle pelagics will be considered in depth; and g) analysis of assessment and management methods.