Links to Online Videos of Good-Quality Lectures by Mathematicians I am not trying to do anything new here, but I would like to start an activity that has been going on at Math Overflow. Hopefully, people here at the Math StackExchange can also enjoy the good stuff.
If you know of any online videos of good-quality lectures by mathematicians, whether they are about some seminal topic, an introduction to some new exciting idea, a re-exploration of a well-known area, or simply about how they do mathematics (like Paul Halmos), please post a link here with a brief description of the contents of the video.
Hopefully, we can start a video bank here! Thank you very much for your contribution!

The audio isn't great but Serre's "Writing Mathematics Badly" is classic. It's a discussion of what is and what is not good mathematics notation, style, etc. It also has quite a bit of humor mixed in:
Part 1
Part 2
Part 3
