An adoption dossier, is a set of legal documents. An adoption dossier is often spoke about in regards to an international adoption. Ask the adoption agency you've chosen to work with for a list of needed paperwork required by your country of choice. It's also a good idea to ask for two or three copies of your marriage license, birth certificates, and other legal documents. You will also need them for your adoption home study.
Don't count on being able to use the same marriage license for your dossier that you use for your home study. Some countries will want original documents only. If you plan on adopting more than one child, some countries require a dossier for each child. So plan on erring on the side of caution and get more than one copy. You never know when you might need them.

