This book constitutes the proceedings of the 20th International Conference on Computer Information Systems and Industrial Management Applications, CISIM 2021, held in Ełk, Poland, September 24–26, 2021. The 38 papers presented together with 1 invited speech and 3 abstracts of keynotes were carefully reviewed and selected from 69 submissions. The main topics covered by the chapters in this book are mobile and pervasive computing, machine learning, high performance computing, image processing, industrial management. Additionally, the reader will find interesting papers on computer information systems, biometrics, security systems, and sensor network service. The contributions are organized in the following topical sections: biometrics and pattern recognition applications; computer information systems and security; industrial management and other applications; machine learning and artificial neural networks; modelling and optimization, and others. Chapter 24 "A first step towards automated species recognition from camera trap images of mammals using AI in a European temperate forest" is published open access under a CC BY license (Creative Commons Attribution 4.0 International License).