Who is responsible for all estimates in the Product Backlog?

The responsibility for all estimates in the Product Backlog lies with the Development Team. This is because the Development Team possesses the technical knowledge and expertise required to assess the complexity and effort involved in completing the different items in the backlog. By providing estimates, the Development Team ensures that the estimates are realistic and grounded in their understanding of the work needed to deliver the product increment.

The Product Owner collaborates closely with the Development Team to clarify the backlog items and express the desired outcomes, but it is ultimately the Development Team that performs the estimation based on their skill set and experience. The Scrum Master serves as a facilitator and coach for the team, but they do not directly contribute to estimating backlog items. Likewise, stakeholders can provide valuable input regarding priorities and requirements, but they do not engage in the estimating process. Thus, all estimates are a function of the Development Team's expertise and should reflect their consensus on the effort needed for each backlog item.