Virus diseases are a major yield reduction factor in bean production. Among the diseases infecting beans, Bean common mosaic disease(BCMD) caused by Bean common mosaic virus (BCMV) and Bean common mosaic necrosis virus (BCMNV) is most widespread. Yield loss due to BCMV and BCMNV ranging between 6 and 98% in some fields has been reported. Much work has not been done on the status of BCMV and BCMNV in Kenyan agro ecological zones (AEZs). This book explores diagnostic survey for BCMD in major bean growing areas of western Kenya and characterization of its causal agents BCMV and BCMNV. Extensive surveys for incidence and severity of BCMD in bean growing areas of western Kenya and scoring scales have been done. Characterization of BCMV and BCMNV isolates using differential cultivars is also explored. Methods of determining susceptibility of popular bean varieties to the virus isolates are also outlined in details.