This chapter focuses on specific diseases marked by notable vascular wall abnormalities. Hereditary hemorrhagic telangiectasia (HHT), formerly called Osler-Weber-Rendu syndrome, is a rare autosomal dominant vascular disorder characterized by mucocutaneous and visceral telangiectasias that affect multiple organ systems, including the skin, liver, brain, and lungs. HHT is the second most common inherited bleeding disorder after von Willebrand disease, caused by loss-of-function mutations in genes that encode proteins involved in endothelial signaling pathways essential for vascular stability and integrity. Manifestations of HHT include recurrent nosebleeds and gastrointestinal bleeding, which can lead to iron deficiency. Henoch-Schönlein purpura (HSP), also known as IgA vasculitis (IgAV), is a small-vessel vasculitis characterized by purpuric rashes, often involving the joints, gastrointestinal tract, and kidneys. Its histological hallmark is leukocytoclastic vasculitis (LCV) with IgA deposits. Although HSP is more common in children, adults typically experience more severe symptoms and have a worse prognosis.