These are links to catalogues, educational web sites , online reference materials, professional agencies you might be interested in.

• Giotto on wikipedia.org
• Giotto Di Bondone - The complete works by <a href="http://www.giottodibondone.org">Giotto Di Bondone</a> Biography, user comments, eCards, more than 500 images of paintings, slideshow and more!
• catalogue of complete works, biography, etc.
• Article on Early Renaissance tempera techniques
• Authentication techniques