You can now request a review explicitly from collaborators, making it easier to specify who you'd like to review your pull request.
You can also see a list of people who you are awaiting review from in the pull request page sidebar, as well as the status of reviews from those who have already left them.
Pending requests for review will also show in the merge box. They do not affect mergability, however, so you can still merge your pull request even if you are still awaiting review from another collaborator.
Learn more about requesting reviews in our Help docs.