An overview of the present knowledge of the family Stenasellidae (Asellota: Aselloidea) is presented, focusing on the success of recent investigations of the underground waters of Oman (SE Arabian Peninsula). We take up the discovery of four new species of the genus Stenasellus : small species adapted to interstitial biotopes and larger ones typical of karstic aquifers. These species cohabit in ecotones between interstitial and karstic water systems. The putative presence of stenasellids in hypogean waters of Yemen and South Iran is suggested. The taxonomic status of the genus Stenasellus auct. needs to be reconsidered in the future A scenario to explain the origin and the presence of these Isopods in continental underground waters is advanced.