MongoDB can generate globally unique identifiers, but sometimes an application may need to create custom unique identifiers. To address this, MongoDB provides several approaches for generating unique identifiers, including using ObjectID, a single counter document, multiple counter documents, randomly choosing a unique identifier and retrying if it is already assigned, or using a standard UUID algorithm. Each approach has its benefits and drawbacks, such as potential bottlenecks with the single counter document approach, the need to define the number of counter documents with the multiple counter documents approach, and the use of 128-bit UUIDs which may result in slightly larger documents. The choice of approach depends on the specific requirements of the application.