It is often noted anecdotally that Danish is difficult to learn as a second language but recent research suggests that Danish is hard to learn even as a native language; e.g., with regard to vocabulary and past tense morphology (Bleses et al., 2008, 2011). Crucially, these problems appear to persist into adulthood: There are longer pauses between utterances in adult Danish conversations than in other languages (Stivers et al., 2009). Moreover, having Danish as a native language appears to make it harder to learn other languages such as English (Delsing & Åkesson, 2005). From a scientific perspective, the difficulty of learning and using Danish poses a major puzzle given that all languages are typically assumed to be equally easy to learn and use. Why is Danish so hard to learn and use? Tantalizing initial data suggest that the unusual sound structure of Danish—characterized by a large vowel inventory, weakening of consonants to vowel-like sounds, and frequent reductive processes—makes it difficult to detect where words begin and end, to perceive inflectional endings, and perhaps even to acquire the grammar. Does the problem of learning Danish extend beyond the early acquisition of vocabulary and morphology to other aspects of language use? Over the past 15 months, we have investigated Danish at the phonological, sentential, and dialogue levels, taking advantage of the natural experimental control afforded by conducting parallel sets of experiments in Denmark and Norway. Norwegian provides an ideal control because its vocabulary, morphology, and grammar are practically identical to Danish but its phonology differs: Norwegians tend to pronounce their consonants. This theme session will report on our results in three separate talks, followed with a critical discussion lead by Ewa Dąbrowska (who is not associated with the project). In the first paper, Trecca et al. present results from story comprehension experiments suggesting that the opaque Danish phonology may make Danes rely more on contextual information in real-time processing of auditorily presented sentences. The second paper by Dideriksen et al. explores the potential impact of Danish phonology on dialogue-level communication. The results point to differences across both spontaneous and task-related conversations in how Danes and Norwegians use repair, backchanneling, and alignment. In the third paper, Ishkhanyan et al. report on two studies investigating phonological processing in Danes and Norwegians. Performance on both nonword repetition and categorical perception tasks reveal possible differences across language users in phonological representation and the use of context. Ewa Dąbrowska leads a final discussion of how the results not only provide insights into the peculiar nature of the Danish language and how it affects learning and use, but also may throw new light on deep theoretical questions in the language sciences relating to language learnability, complexity, and linguistic universals.