In-depth background knowledge on detecting and identifying plant diseases is essential for helping to raise healthy crops. This chapter provides an overview of historical developments in understanding and monitoring plant diseases, including the invention of light and electronic microscopes, Koch’s postulates on proof of pathogenicity, the development of enzyme-linked immunosorbent assay (ELISA) and the use of nucleic acid-based tools for detection of plant pathogens. The chapter then considers the epidemiology of plant diseases and disease diagnosis in plants. Finally, the chapter looks ahead to future research trends in this area.