Introduction to the Special Issue on QEST 2021

The International Conference on Quantitative Evaluation of SysTems (QEST) is the leading forum on evaluation and verification of computer systems and networks, through stochastic models and measurements. QEST covers topics including classic measures involving performance and reliability, as well as quantification of properties that are classically qualitative, such as safety, correctness, and security. QEST welcomes measurement-based studies as well as analytic studies, diversity in the model formalisms and methodologies employed, as well as development of new formalisms and methodologies. In short, QEST aims to encourage all aspects of work centered around creating a sound methodological basis for assessing and designing systems using quantitative means. This special issue consists of five articles extending earlier versions presented at QEST 2021, the 18th edition of the conference, which was hosted in Paris but virtually held August 23 through 27, 2021. A selection of the top-ranked conference papers was chosen by the chairs, and the authors were invited to submit an extended version to this special issue. The journal review process included both members of the QEST program committee and additional reviewers who were not involved in the conference refereeing process. The resulting collection of articles comprises exciting developments in the areas of system verification and performance or reliability analysis. In the contribution titled “Optimizing Reachability Probabilities for a Restricted Class of SHA via Flowpipe Construction,” da Silva, Schupp, and Remke