Boundaries are defined as invisible lines that help keep the “good” in and the “bad” out, but are also permeable enough to allow some things to pass through. When defining them in the context of relationships, they are guidelines, rules, or limits that regulate communication and contact with others. Typical boundaries include physical, mental, emotional, and sexual limits set on relationships to regulate those relationships and protect people from abuse or manipulation. Clear boundaries help develop responsibility and self-control while still helping people feel cared for, connected, and safe. This entry discusses the development of boundaries in childhood, how the idea of boundaries is utilized in structural family therapy, and concludes with recommendations for maintaining healthy boundaries.