His focuses include cognitive and social processes in language use; interactive processes in conversation, from low-level disfluencies through acts of speaking and understanding to the emergence of discourse; and word meaning and word use. Clark is known for his theory of "common ground": individuals engaged in conversation must share knowledge in order to be understood and have a meaningful conversation (Clark, 1985). Together with Deanna Wilkes-Gibbs (1986), he also developed the collaborative model, a theory for explaining how people in conversation coordinate with one another to determine definite references. Clark's books include Semantics and Comprehension, Psychology and Language: An Introduction to Psycholinguistics, Arenas of Language Use and Using Language."