In this article, we discuss the importance of indexation in achieving performance with MongoDB. The author emphasizes that not defining appropriate indexes is the primary issue that support teams face with users. To use indexes effectively, we need to follow best practices such as using composite indexes, following the ESR rule, and utilizing full-text search. We also need to be cautious when indexing fields with low cardinality and avoid unnecessary index maintenance costs. Additionally, we can optimize our indexes by using partial indexes, multi-key indexes, and leveraging MongoDB's built-in optimization features like WiredTiger. The author concludes by highlighting the importance of monitoring and analyzing index usage and providing recommendations for improvement.