Molecular biology has emerged as one of the most important branches of biological sciences helping them in achieving rapid and significant advancements. This branch has been able to provide the essential information on the basic cell behavior pattern. The structure and function of nucleic acids (DNA & RNA) and proteins were extensively examined. The individual molecules function generally as components of complex gene expression mechanism in metabolic pathways. This book presents updated and comprehensive information of molecular aspects of plant pathogens in to nineteen articles. Different plant pathogens like Bipolaris, Phytophthora infestans, Macrophomina phaseolina, Fusarium spp., Ganoderma, Plasmopara halstedii, Pyricularia grisea, Pythium aphanidermatum were characterized by different methods. However the reuse of agarose gel in molecular studies is beneficial to all laboratories in this area. The review article of Phtoplasma, Plant viruses, Vegetative Compatibility Groups (VCGs) in fungi, Fusarium, Trichoderma were highly effective and adds the recent information which is beneficial to researchers. The recent advances in diagnosis of plant pathogens by different methods are included in this book. The different authors deserved excellent job through involving recent references in respective articles and brought very useful information for workers in molecular plant pathology.