These guys are working on the top cap west antenna of the John Hancock Center — a 1,127-foot-high (344 metres) building located in Chicago. I know they are wearing the mandatory security equipment and it's very unlikely that they could fall, but watching them do their job makes my stomach clench.
In case you are wondering what are these guys doing up there. This is what—according to the youtube description:
Making notch cut of the top cap west antenna on the John Hancock building so that mast can be installed to start removal of the top sections.