ASP? Which ASP? There's classic ASP (filename.ASP) and ASP.Net (filename.ASPX)
What level of integration? If you just want the pages to be part of the SharePoint URL space, upload the files (either .ASP or .ASPX) to the LAYOUTS folder of the 14 hive. If you want the pages to have the SharePoint look and feel, then you will need to
move your code into a SharePoint content page linked to a SharePoint master page. The most flexible way to bring your custom application into SharePoint is to roll your coded into ASP.NET web parts.
Tell us more about what you want to do.
Mike Smith TechTrainingNotes.blogspot.com