The decade-old Internet search result pages, manifested in the document-centric form of "ten blue links," are no longer sufficient for Internet search users. Many studies have shown that when users are ushered off the conventional search result pages through blue links, their needs are often partially met at best in a "hit-or-miss" fashion. To tackle this challenge, we have designed Bing (www.bing.com), Microsoft's search engine, to not only navigate through web sites but also engage with users to clarify their intent to facilitate the task completion. Powering this new paradigm is the Bing Dialog Model that consists of three building blocks: an indexing system that comprehensively collects information from the web and systematically harvests knowledge, an intent model that statistically infers user intent and predicts next action based on the harvested knowledge, and an interaction model that elicits user intent through mathematically optimized presentations of web information and knowledge that matches user needs. In this talk, I'll describe Bing Dialog Model in details and demonstrate it in action through innovative features, in particular by applying entity and intent understanding to proactively assist user query formulation for task completion.