This book constitutes the refereed proceedings of the 17th Conference on Fuzzy Logic and Intelligent Technologies in Nuclear Science, FLINS 2026, and 21st International Conference on Intelligent Systems and Knowledge Engineering, ISKE 2026, held in Sydney, NSW, Australia, during July 15–19, 2026.The 152 full papers and 8 short papers presented in this book were carefully selected and reviewed from 275 submissions. These papers have been organized in the following topical sections:Part I: Fuzzy Machine Learning and Decision MakingPart II: Knowledge Engineering, LLMs, and ReasoningPart III: Computer Vision and Medical ImagingPart IV: Graph Neural Networks and Relational IntelligencePart V: Trustworthy, Federated, and Robust Learning Systems