This article explains what a Entity-Relationship Diagram (E-R Diagram) of a Library Management System is and how it is used in software development. An E-R Diagram is a graphical representation of entities and their relationships. It shows how data is organized, make it easier to plan databases, and helps developers to understand the data structures they need to develop. An E-R Diagram of a Library Management System typically consists of entities like Books, Patrons, Librarians, and Checkouts and their relationships. This Diagram provides a good basis for developers to build a Library Management System that meets the needs of the organization it is being built for.