The aim of this article is to establish whether the structure of morphologically complex words in English is flat or hierarchical and, in case of the latter possibility, whether it is left- or right-branching, and whether the branching direction is consistent throughout the lexicon. Six criteria (three phonological, one morphological, one lexical, and one semantic) are introduced as arguments bearing on the internal structure of words. These criteria are applied to the complete set of prefix-stem-derivational suffix sequences in English. The overall evidence clearly weighs in favour of the hierarchical right-branching model. Right-branching is argued to be a concomitant of translating meaning into form. No branching-direction preference emerges on the semantic criterion. Right-branching is relatively weak on the lexical criterion, stronger on the morphological criterion, and strongest on the phonological criteria. Left-branching is observed to play a certain role in words with unproductive prefixes and is understood as a symptom of demorphologization. The branching direction is found to be largely, though not entirely, consistent. The six criteria generate a low percentage of conflicting outcomes, suggesting that branching paradoxes are a rather uncommon phenomenon. The reason for hierarchical right-branching structure in English is claimed to lie in the processing advantage that it incurs. This explanation applies not only to the morphological but also to the syntactic and phonological levels. Processing ease thus leads to crosslevel harmony in the English language.