Historically, debates over relationships between lexical form and meaning have been dominated by views of arbitrariness, until more recent research revealed a different perspective in which non-arbitrary mappings play an important role in the makeup of a lexicon. It is now clear that phoneme-sound symbolism contributes to non-arbitrariness (iconicity) of words, which is present in many forms and degrees in different languages. Attempts have been made to provide a mechanistic explanation of the phenomenon, and these theories largely centre around cross-modal correspondences. The multimodal communicative system, to which iconicity is integral, has important phylogenetic and ontogenetic advantages, facilitating language learning, comprehension, and processing. Despite its numerous advantages, iconicity must compete with arbitrariness, forcing language systems to balance the competing needs to link linguistic form with human experience and to ensure an effective signal. On balance, iconicity should be viewed as a core principle of language, and not merely a marginal phenomenon.