This document defines an inventory of building blocks conceptually associated with different types of assessments of information and communication technology (ICT) trustworthiness. These assessments apply to areas such as governance, risk management, security evaluation, secure development lifecycle (SDL), supply chain integrity and privacy. This document also defines an ontology that organizes these building blocks and provides instructions for using the inventory of building blocks and the ontology. Formalizing the types, categories, and structural characteristics of building blocks in the area of ICT trustworthiness assessment aims to increase efficiency and improve future harmonization in standards development and their use. Building blocks can refer to structural components as well as semantic components. These components can be connected to a variety of concepts and activities related to trustworthiness assessments, including process related, such as traceability or elements of assessment methodologies.