When faced with a library system migration, it can be daunting to know where to begin with data cleanup. In many cases, there is a long list of tasks to tackle before switching to a new system and not enough time to perform them all. Therefore, it is important to target your efforts to the most impactful areas. In this presentation, I will provide guidance on deciding where to focus your available time for metadata cleanup. I will present metadata assessment criteria that will help you know what to be looking for in your data that could potentially cause issues during a migration. Finally, I will share analysis techniques and examples to show how to look for those issues in your metadata, drawing on my experience performing this work for our library's recent migration.