Book excerpt: This edited book provides the readers with the concepts and in-depth knowledge of plant disease assessment and conventional and modern technologies that aid in precise and accurate phytomathometery. This book discusses the evolution of plant disease assessment procedures from the primary visual estimation-based assessment to modern approaches, their practical application for reliable disease quantification, yield loss estimation, and the efficacy of disease control strategies for sustainable crop protection. Significant information is provided on the major aspects of the topic, including remote sensing, imaging techniques, molecular phytopathometery, microarray, and immunotechnology. The book helps plant scientists, plant pathologists, practitioners, researchers, and students in disease quantification, developing predictive models for plant disease epidemics, assessing crop losses, and the magnitude of plant disease control methods. This book describes the classical plant disease assessment methods based on visual observations. It Provides information regarding the modern and emerging technologies in Phytopathometery, precision, and accuracy. This book also discusses the application of disease assessments in predictive models, disease warning systems, expert systems, and decision support systems in applied plant pathology.