The QUDT, or 'Quantity, Unit, Dimension and Type' collection of ontologies define the base classes properties, and restrictions used for modeling physical quantities, units of measure, and their dimensions in various measurement systems. QUDT provides a unified model of measurable quantities, units for measuring different kinds of quantities, the numerical values of quantities in different units of measure and the data structures and data types used to store and manipulate these objects in software. This OWL schema is a foundation for a basic treatment of units. Originally developed by TopQuadrant for the NASA Exploration Initiatives Ontology Models (NExIOM) project, they now form the basis of the NASA QUDT Handbook. QUDT aims to improve interoperability of data and the specification of information structures through industry standards for Units of Measure, Quantity Kinds, Dimensions and Data Types.