The sprint retrospective is an opportunity to inspect and adapt the process as it is a time to reflect on the process. It is needed that the full Scrum team attends. This includes all members of the development team (includes everyone who is designing, building, and testing the product), the Scrum Master, and the product owner. Since the Scrum Guide also states that the Scrum team = product owner + Scrum Master + development team, we can deduce that (officially at least) the product owner is allowed to attend the retrospective. This is the ideal scenario. However, sometimes, the teams might not wish to include the product owner as it might hinder their discussion. If there is a lack of trust between the product owner and the development team, or there is a low level of safety so that speaking candidly isn’t comfortable, perhaps the product owner should not attend until the Scrum Master can help coach those involved towards creating a safer, more trusting environment. Anyone else outside of the immediate scrum team, and particularly managers of team members, should not be invited to participate.