To take part in the PCD network's Mobility Programme, M.A. and PhD students require an informal letter from a mentor offering to supervise their research during their stay at Harvard University. A mentor should be a humanities professor at the host university whom you choose and contact on your own before your stay. An overview of the departments and institutes in the humanities at Harvard can be found here.
A mentor must not necessarily be a professor working in your own field. Nonetheless, you will be affiliated with the institute of your mentor during your stay.
PostDocs are asked to show that they have already established contact with researchers at Harvard University (e.g., with an extract from an email).