Basically, 40% of the episodes of the first six years seasons of Doctor Who (1967-73 1963-69, First and Second Doctors) and a few more until 1978 only survive as audio clips and still frames. Finding actual footage would be great to fill these gaps in the serials (Doctor Who stories come in groups of episodes).
Edits: Doctor Who started at 1963, not 1967. Also spelling and an extra line.