Technical Abstract: Transmissible viral proventriculitis (TVP) is an infectious, transmissible viral disease of chickens that results in enlarged, fragile proventriculi. TVP is characterized by specific microscopic changes that include glandular epithelial cell necrosis, ductal epithelial cell hyperplasia, and lymphocytic inflammation. Experiments show that TVP can be transmitted to both broiler and specific pathogen free Leghorn chickens by a filterable agent that appears to be a non-enveloped virus. Differential diagnoses for TVP include proventriculitis caused by the ingestion of toxins such as biogenic amines, copper sulfate, or mycotoxins, or proventriculitis associated with the finding of infectious agents that include bacteria, fungi, cryptosporidia, and viruses. In contrast, proventricular dilatation is characterized by an enlarged, but thin-walled proventriculus with no microscopic evidence of inflammation. Chickens affected by TVP may cost more to produce than unaffected chickens due to poor food digestion, poor feed conversion, and stunted growth. Rupture of the proventriculus or gastric isthmus during carcass evisceration results in carcass contamination and significant economic loss. This book chapter provides general information on incidence and distribution, etiology, epizootiology, clinical signs, lesions, and diagnosis of proventriculitis in poultry.