This special issue presents the state-of-the-art in implemented, general-purpose Natural Language Processing (NLP) systems that use nontrivial Knowledge Representation and Reasoning (KRR). These systems use full-scale implementations of traditional KRR techniques as well as some newer knowledge-related processing mechanisms that have been developed specifically to meet the needs of natural language processing. The papers cover a wide range of natural language inputs, knowledge and formalisms, application domains and processing tasks, illustrating the key role that knowledge representation plays in all types of NLP systems.