In the face of a growing workload and dwindling resources, the US National Library of Medicine (NLM) created the Indexing Initiative project in the mid-1990s. This cross-library team’s mission is to explore indexing methodologies that can help ensure that MEDLINE and other NLM document collections maintain their quality and currency and thereby contribute to NLM’s mission of maintaining quality access to the biomedical literature. The NLM Medical Text Indexer (MTI) is the main product of this project and has been providing indexing recommendations based on the Medical Subject Headings (MeSH) vocabulary since 2002. In 2011, NLM expanded MTI’s role by designating it as the first-line indexer (MTIFL) for a few journals; today the MTIFL workflow includes about 100 journals and continues to increase. Due to a close collaboration with the Index Section at NLM, MTI continues to grow and expand its ability to provide assistance to the indexers. This paper provides an overview of MTI’s functionality, performance, and its evolution over the years.