The Owl, The Pussycat and the Investiture Controversy
- Professor emeritus Lynn H. Nelson explains the Investiture Controversy in medieval dialogue form.
- http://www.vlib.us/medieval/lectures/investiture.html

The Investiture Conflict
- Lecture by Lynn Nelson in the Online Reference Book for Medieval Studies.
- http://the-orb.net/textbooks/nelson/investiture.html

Conflict of Investitures
- Article from the Catholic Encyclopedia.
- http://www.newadvent.org/cathen/08084c.htm

Empire and Papacy
- An introduction and collection of sources from the Medieval Sourcebook on the relationship between the Pope and the Holy Roman Emperor.
- http://www.fordham.edu/halsall/sbook1l.html