Fungal diseases of lentil such as Fusarium wilt (Fusarium oxysporum f. sp. lentis), Stemphylium blight (Stemphylium botryosum f. sp. lentis), and Ascochyta blight (Ascochyta lentis) are the major biotic constraints to lentil production and reported to cause significant yield and quality reduction in lentil in most of the lentil growing areas. Additionally, there are several other diseases such as Botrytis gray mold (Botrytis fabae and B. cinerea), rust (Uromyces viciae-fabae), Sclerotium rot (Sclerotium rolfsii), and anthracnose (Colletotrichum truncatum) which are important in lentil growing areas with environmental condition favoring the infection process. There are numerous viral diseases including Bean leaf roll virus (BLRV), Bean yellow mosaic virus (BYMV), Beet western yellows virus (BWYV), Cucumber mosaic virus (CMV), Faba bean necrotic yellows virus (FBNYV), Pea enation mosaic virus-1 (PEMV-1), Pea seed-borne mosaic virus (PSbMV), Pea streak virus (PeSV), Broad bean stain virus (BBSV), and subterranean clover red leaf virus (SCRLV) that affect lentil. However, they are reported to be minor diseases as the yield losses are not as high as in the case of fungal diseases. This chapter discusses several economically important lentil diseases with special reference on the distribution and impact of the disease, the biology and epidemiology of the pathogen, and integrated disease management approaches.