Very useful. I hooked on catalogs when working with big DTDs such as Docbook. Validate against huge DTD or schema loading it from the Web isn't good idea and catalogs here is a feature one cannot work without. It's a shame there is still no .NET implementation. Basically that's on my to-do list for almost a year, still close to bottom :( There was some showstopper related to PUBLIC identifier, but I don't remember exactly what the the problem was. It's still tempting to implement it. Probably that's going to be my next pet project after I finish EXSLT article I'm writing. Anybody interested to participate?