Tagging on Flickr has grown exponentially over time as more users have adopted the practice of adding tags to their photos on the site. Early tags tended to be simple labels like "birthday" or "vacation" but tags have become increasingly sophisticated as users describe concepts and annotate relationships between people, places, and events. The growing use and complexity of tags on Flickr now presents the greatest challenge of scaling the system to effectively support exploration of the vast tag space and increasing surface area for search and discovery.