This book constitutes the refereed conference proceedings of the 14^th International Conference on Performance Evaluation Methodologies and Tools, VALUETOOLS 2021, held in October 2021. Due to the safety concerns and travel restrictions caused by COVID-19, VALUETOOLS took place online in a live stream. VALUETOOLS 2021 aims to provide a world-leading and multidisciplinary venue for researchers and practitioners in diverse disciplines such as computer science, networks and telecommunications, operations research, optimization, control theory and manufacturing.  The 16 full papers were carefully reviewed and selected from 32 submissions and focus on methodologies and practices in modelling, performance evaluation and optimization of complex systems.