The vowel shortening known as Osthoff's Law (*VRC > VRC) has been an established rule of early Greek phonology for over a century. Despite sporadic objections, it is generally accepted as a genuine historical shortening. However, developments in Indo-European and Greek linguistics have meant that Osthoff's examples are now much less certain: it is hard to prove that they ever contained a long vowel. The degree to which this affects the corpus of Osthoff shortenings is demonstrated by a detailed investigation of the forms involved. Osthoff' s Law is closely tied in with a problematic area of PIE phonological reconstruction: the original syllabification and subsequent development of *-VHRCsequences. The extent to which these two issues depend upon each other is demonstrated and discussed. It is then shown that OL has further importance for other controversial issues of PIE reconstruction. The Classical Greek forms which apparently escaped Osthoff shortening can reveal more about the details of this sound law. A particularly important case is the set of ' phonologically immune' sequences, which provide a key insight into the real nature of Osthoff's Law. The phonological issues are considered, beginning with the previous formulations and suggested motivations, and it is concluded that Osthoff's Law is best seen as one repair strategy of a stricture against trimoraic syllables. The trigger for OL can be identified as a change in the accent and weight assignment system of early Greek, which would not otherwise be recoverable.