An illustrated key to separate all the polychaete families reported to present date within the limits of the Chilean sea is presented. For practical purposes the arbitrary grouping into Polychaeta Errantia and Polychaeta Sedentaria has been followed. The first group is represented by 27 families and the second group comprises 26 families. In addition, a brief description of some morfological features used in taxonomy of polychaetes and suggestions on fixation and preservation of specimens are given.