Intent: This question, often worded as "significant accomplishment," ranks among the most predictable and important things you'll be asked. Interviewers want to hear how you tackled something big. It is vital you give them an organized, articulate story.
Context: This is a behavioral question -- meaning you're being asked to talk about a specific example from your professional history. Pick an example or story about how you handled a major project that is both significant to you and rich in detail.
Response: Set up the story by providing context. Recount the situation and your role in it. Next, discuss what you did, including any analysis or problem solving, any process you set up and obstacles you had to overcome. Finally, reveal the outcome and what made you proud.