Top-Down Estimation is a project management technique used to estimate the overall cost or duration of a project by starting with the big picture. This method involves breaking down the project into smaller components, but the initial estimates are based on the overall project goals and objectives. It is often quicker and less detailed than other estimation methods. This approach is useful in situations where detailed information is not available, allowing teams to make informed decisions based on high-level insights. However, it may lack accuracy if the initial assumptions are not well-founded, making it essential to validate estimates as the project progresses.