From Verification to Explanation (Track Introduction) -- An Algorithm to Compute a Strict Partial Ordering of Actions in Action Traces -- TraceVis: Towards Visualization for Deep Statistical Model Checking -- Engineering of Digital Twins for Cyber-Physical Systems -- Understanding Digital Twins for Cyber-Physical Systems: A Conceptual Model -- Uncertainty quantification and runtime monitoring using environment-aware digital twins -- Designing Distributed Control with Hybrid Active Objects -- Towards a Digital Twin - Modelling an Agricultural Vehicle -- Digital Modelling in the Railways -- Engineering a Digital Twin for Manual Assembling -- Towards Digital Twins for Knowledge-driven Construction Progress and Predictive Safety Analysis on a Construction Site -- Software Verification Tools (Track Introduction) -- Benchmarking Open-Source Static Analyzers for Security Testing for C -- Verification of Liveness and Safety in BP using BPjs -- On Correctness, Precision, and Performance in Quantitative Verification -- Every Component Matters: Generating Parallel Verification Benchmarks with Hardness Guarantees.

The four-volume set LNCS 12476 - 12479 constitutes the refereed proceedings of the 9th International Symposium on Leveraging Applications of Formal Methods, ISoLA 2020, which was planned to take place during October 20-30, 2020, on Rhodes, Greece. The event itself was postponed to 2021 due to the COVID-19 pandemic. The papers presented were carefully reviewed and selected for inclusion in the proceedings. Each volume focusses on an individual topic with topical section headings within the volume: Part I, Verification Principles: Modularity and (De-)Composition in Verification; X-by-Construction: Correctness meets Probability; 30 Years of Statistical Model Checking; Verification and Validation of Concurrent and Distributed Systems. Part II, Engineering Principles: Automating Software Re-Engineering; Rigorous Engineering of Collective Adaptive Systems. Part III, Applications: Reliable Smart Contracts:State-of-the-art, Applications, Challenges and Future Directions; Automated Verification of Embedded Control Software; Formal methods for DIStributed COmputing in future RAILway systems. Part IV, Tools and Trends: From Verification to Explanation; Engineering of Digital Twins for Cyber-Physical Systems; Software Verification Tools.