ABSTRACTModern Icelandic presents phonological oppositions between two series of stop consonants and two series of vowels. Unlike in Old Icelandic, these are not phonetically characterized by voice in the former case or length in the latter, and the modern distinguishing features have commonly been described in both cases as ‘tenseness’ (versus ‘laxness’). There has, however, been no adequate definition of this feature, and the distinctions involved are fully describable in terms of various more traditional and observable processes. There is in fact no one feature common to the consonantal and vocalic oppositions in question; and even within each of these some problems arise when one attempts to find a single defining criterion. Various possible solutions are examined and evaluated. Some consideration is also given to the relevant historical and dialectal dim