Over the last two decades, advances in the design, miniaturization, and analytical capabilities of portable X-ray fluorescence (pXRF) instrumentation have led to its rapid and widespread adoption in a remarkably diverse range of applications in research and industrial fields. The impetus for this volume was that, as pXRF continues to grow into mainstream use, analysts should be increasingly empowered with the right information to safely and effectively employ pXRF as part of their analytical toolkit. This volume provides introductory and advanced-level users alike with readings on topics ranging from basic principles of pXRF and qualitative and quantitative approaches, through to machine learning and artificial intelligence for enhanced applications. It also includes fundamental guidance on calibrations, the mathematics of calculating uncertainties, and an extensive reference index of all elements and their interactions with X-rays. Contributing authors have provided a wealth of information and case studies in industry-specific chapters. These sections delve into detail on current standard practices in industry and research, including examples from agricultural and geo-exploration sectors, research in art and archaeology, and metals industrial and regulatory applications. As pXRF continues to grow in use in industrial and academic settings, it is essential that practitioners continue to learn, share, and implement informed and effective use of this technique. This volume serves as an accessible guidebook and go-to reference manual for new and experienced users in pXRF to achieve this goal.