The seemingly idiosyncratic distribution of glides in the weak verb stems of Classical Arabic is given a coherent analysis as the consequence of constraint interaction, as defined by Optimality Theory. At the core of the analysis are two constraint rankings that determine the vowels of the verb stem. One ranking, which ensures harmonic parsing of a low vowel over high vowels, is based on input/output faithfulness; the other ranking, which ensures harmonic parsing of high vowels over a low vowel, is based on intercandidate faithfulness, as defined by Sympathy Theory. These constraint rankings interact with generally defined markedness constraints to account for glide distribution in all measure I verb forms without specific reference to morphological contexts. As a result, the complex distribution of glides in Arabic is not typologically anomalous.