This book constitutes the refereed proceedings of the 21st International Conference on Text, Speech, and Dialogue, TSD 2018, held in Brno, Czech Republic, in September 2018. The 56 regular papers were carefully reviewed and selected from numerous submissions. They focus on topics such as corpora and language resources, speech recognition, tagging, classification and parsing of text and speech, speech and spoken language generation, semantic processing of text and search, integrating applications of text and speech processing, machine translation, automatic dialogue systems, multimodal techniques and modeling.