Knowledge representation (KR) is a field of artificial intelligence to convey information about the world to a computer to solve complex tasks. This book is a fresh viewpoint on KR and ontology engineering, informed by a variety of projects over the past dozen years, and guided by the ideas of Charles Sanders Peirce (1839–1914), an American logician, scientist, mathematician, and philosopher of the first rank. Peirce’s writings over a half-century spanned the nature of knowledge to metaphysics and cosmology. Among his profound contributions, Peirce’s universal categories of Firstness, Secondness, and Thirdness distill the mindset that guides this book. Attributes are a Firstness in the universal categories; Secondness captures all events, entities, and relations; and Thirdness generalizes the types, context, meaning, and methods we use to make sense of the world. The book uses an open-source knowledge artifact, KBpedia, to illustrate many of its points. The artifact combines information from multiple public knowledge bases, notably Wikipedia and Wikidata, under the KBpedia Knowledge Ontology, a knowledge graph organized by the universal categories. The order of presentation follows Peirce’s logic triad of grammar (1ns), logics and tools (or critic) (2ns), and methods (or methodeutic) (3ns). Peirce scholars know how infused his writings are with “threes.” Understanding, inquiry, and knowledge require this irreducible structure; connections, meaning, and communication depend on all three components standing in relation to one another. Peirce offers a responsive way to model context and perspective, and to represent human knowledge so AI-powered computers can organize, index, reference, and cross-check digital information in any form.