The Isopod and Tanaidacean Crustacea of the Netherlands have not been treated as a whole since 1889, when Hoek published the second part of his Crustacea Neerlandica dealing with the Isopoda and Amphipoda. Hoek in this paper mentioned 25 species of Isopoda, 9 of which are marine forms, 1 a freshwater form and 15 terrestrial, no Dutch Tanaidacea were known to Hoek. At present we know 49 species of Dutch Isopoda, of which 20 are marine, 2 are freshwater forms and 27 are terrestrial, furthermore three species of Tanaidacea are known from this country. The present paper is an enumeration of all Dutch species of the two above mentioned groups, with their distribution within our country and, if necessary, with remarks on their synonymy and ecology. A volume on the Isopoda and Tanaidacea for the series "Fauna van Nederland", written in the Dutch language is now ready for the press and will be published in the course of time; in this volume a complete bibliography of the Dutch Isopoda is given, so that in the present paper only the most necessary references are made. The Isopoda found in Dutch greenhouses and those belonging to the family Armadillidiidae are treated here only summarily, as they have been dealt with more extensively in previous papers (Holthuis, 1946a, b). The species Limnoria quadripunctata new species, Sphaeroma hookeri Leach, Dynamene bidentata (Adams), Idotea granulosa Rathke, Porcellium conspersum (Koch), Armadillidium pictum Brandt, Athelges paguri (Rathke) and Apseudes talpa (Mont.) are recorded here for the first time as belonging to the Dutch fauna. ISOPODA