Obviously with any migration, you’ll be moving and transferring a lot of data. How much, what kind and where from depends on the kind of migration you’ll be doing.
If you’re moving from another platform (like Skype for Business online or Slack), you’ll need to transfer and convert everything – including user data, files, downloads, passwords, associated apps and any external third party storage information.
If you’re currently only using email as your main communication tool, a Teams migration will involve setting up users and making sure any centrally saved documents or attachments are in a compatible location.
If it’s a tenant-to-tenant migration of Teams – perhaps moving from one system to another, or in the case of a merger or acquisition for example, you’ll need to consolidate all of your Teams data before moving it. This isn’t as simple as it sounds, either; because Teams interacts and integrates with so many other elements of the Microsoft Office 365 suite, resources and data are stored in multiple locations. For example you might have a SharePoint site with communal files and shared documents, an Exchange mail system for users, an Azure system for media files, and OneDrive storage for other business assets.