Artificial analogues of natural-language phonological patterns can often be learned in the lab from small amounts of training or exposure. The difficulty of a featurally-defined pattern has been hypothesized to be affected by two main factors, its formal structure (the abstract logical configuration of the defining features) and its phonetic substance (the concrete phonetic interpretation of the pattern). This paper, the first of a two-part series, reviews the experimental literature on structural effects. The principal finding is a robust complexity effect: Patterns which depend on more features are reliably harder to learn.