In ELF research, ample evidence has been collected to show that communication in (dialogic) ELF interactions works and that it does so in intriguingly creative ways. In a questionnaire survey and an in-depth interview study, simultaneous conference interpreters present a less optimistic view with regard to (monologic) mediated multilingual settings, which are increasingly shaped by a growing number of non-native English-speaking participants. Moreover, the interpreters put the adverse effects of ELF speaker output on their cognitive processing down to the speakers' restricted power of expression. This is paralleled by empirical evidence from ELF speakers in TELF (the Tübingen English as a Lingua Franca corpus and database), who put into perspective their general feeling that they can cope in ELF interactions (which is in line with the ELF study findings mentioned above) by voicing dissatisfaction with their restricted capacity of expressing what they want to convey with the required or desired degree of precision. In a theoretical discussion, the Express-ability Principle is introduced to capture the nature of the human effort for expression (complementary to Bartlett's effort after meaning). In the subsequent presentation, sociocultural and psycholinguistic research sheds light on express-ability in the context of ELF by applying Slobin's Thinking for Speaking (TFS) hypothesis to second-language contexts. It reveals the interface between verbal (L1) thinking and externalized (L2) speech and explains expression-related problems in terms of transfer effects in connection with age of acquisition and linguistic environment. This directs further ELF research into the nature of express-ability towards an examination of production processes, developmental and procedural aspects in early and late bilingual ELF speakers, a shared languages benefit to compensate for cross-linguistic transfer and the (relative) effectiveness of unmediated and mediated ELF communicatio
