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Now that you know the basics of what goes into a package diagram the question becomes when do you use package diagrams. Think of the problem this way, a large and complex project can contain hundreds of classes without someway to organize those classes it becomes impossible to make sense of them of all, kind of like trying to find one particular story within this pile of books. Packages create a structure for your classes or other UML elements by grouping related elements. So instead of a pile of books we have a library where books are organized into sections based on similarities in their topics and you want to do the same thing with your class library. You want to make sure that it's organized, that similar classes are grouped together and that your overview is easy to understand. So for general guidelines use package diagrams when you want to show a high level view of the system. And we might be talking about a high level overview of the requirements or we might be talking about a high level view of the design. Packages will always roll up UML elements and take your diagram to a higher level. Use package diagrams to keep track of dependencies. Again in a large system with hundreds of dependencies for system stability you need to keep track of all of those and organize them and manage them and packages make it much simpler to do that. As you recall the elements in a package diagram basically show packages and their dependencies. So package diagrams let you focus on dependencies. Use package diagrams with a large system to show its major elements and how they relate to one another. Again it gives you a high level overview of the system. And you want to use package diagrams to divide a complex system into modules. So this gives you an idea of when package diagrams are helpful and as I said they're particularly useful for a large and complex systems or to give you a high level overview of the system or some aspect of it.