Disqus has many integrations available which make installation easy. The most popular integrations also come with built-in importing and syncing tools so you can bring in your old comments as well. Before installing, make sure you've registered a Disqus shortname, and this will be used to reference all of your comments and settings.
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You can export your old comments from WordPress and import them into Disqus, or sync comments made in Disqus back to WordPress

The WordPress community is a great place for answers for things Disqus is unable to assist with.

We're currently unable to support import and syncing functionality for Blogger. We recognize that syncing and import functionality is an important feature for many Blogger blogs and we apologize for any pains this disruption in service may cause. We want publishers to have a great experience and hope to restore import and syncing services to Blogger in the future.

Learn more about exporting for backup purposes

If the theme author included Disqus integration, this is how you'd enable it.

Not every theme includes Disqus or integrated it correctly. This is a guide on manual installation.

Integrations that are managed for their platforms as plugins or widgets.

How to customize your Disqus integration on the web, add to mobile apps and more.