Verification and testing: Enabling behaviour tree verification via a translation to BIP -- Verified fault handling for modern board management controllers -- Testing compositionality. Formal models: Correct pattern-based development through refinements and weakest preconditions calculus -- Coq formalization of orientation representation: matrix, Euler angles, axis-angle and quaternion -- Reactive graphs in action. Security and blockchain: Extracting formal smart-contract specifications from natural language with LLMs -- How do asynchronous communication models impact the composability of information flow security?

This book constitutes the refereed proceedings of the 20th International Conference on Formal Aspects of Component Software, FACS 2024, held in Milan, Italy, during September 9-10, 2024. The 7 full papers and 1 short paper included in this book were carefully reviewed and selected from 16 submissions. They are organized in topical sections as follows: verification and testing, formal models, and security and blockchain.