The vascular basement membrane (BM) contributes substantially to blood vessel formation and function. Collagens XV and XVIII, both with properties of collagens and proteoglycans and containing an anti-angiogenic endostatin domain, are common components of vascular BMs. Our mouse models for these collagens have revealed that although they share many structural features, their biological functions are essentially different. For example, lack of collagen XVIII leads to abnormal outgrowth of retinal vasculature and delayed regression of hyaloid vessels, while collagen XV appears to regulate capillary structure and function in the heart and skeletal muscle and lack of this collagen leads to mild myopathic phenotype and may also predispose the mice to cardiomyopathy. Collagen XVIII/endostatin affects both tumor angiogenesis and lymphangiogenesis, and regulates inflammatory response in skin tumors. Finally, our recent mouse studies point out the importance of these collagen molecules in maintaining normal vascular permeability.