Book excerpt: This book constitutes the refereed proceedings of the 16th International Conference on Hybrid Artificial Intelligent Systems, HAIS 2021, held in Bilbao, Spain, in September 2021. The 44 full and 11 short papers presented in this book were carefully reviewed and selected from 81 submissions. The papers are grouped into these topics: data mining, knowledge discovery and big data; bio-inspired models and evolutionary computation; learning algorithms; visual analysis and advanced data processing techniques; machine learning applications; hybrid intelligent applications; deep learning applications; and optimization problem applications.