It kind of depends on the height, work being done, and local/federal regulations.
For example if you were working on a roof that is not very high, you would take much different precautions than if you were cleaning windows on a skyscraper. On the roof, it would probably be sufficient to ensure that the ladder is well placed and that, depending on the pitch of the roof, roof jacks are used. However, when cleaning windows on a skyscraper, you would need to be tied in, using ropes and harnesses.
Federal and local regulations will also play a role, at least in the United States, where there are a number of cases where rope and harness will be required for commercial settings.