[Update: This article was written for Aperture 1.1, so does not mention the more advanced importing features that 1.5 offers]

A more efficient way of getting a lot of images into Aperture than using the Import dialog is to simply drag the folder in. My example photos are in a folder called Photos and open up to a complete hierarchy that looks like this:

Dragging and dropping into the project pane, or onto the Library results in this:

Now all the hierarchy of the folder structure is maintained and Albums automatically created. In each Brown Folder there is an initial Album that contains the images that were not in any sub-folder, for example Images from: Animals. Exactly the same result can be achieved by using the menu item File -> Import -> Folders Into A Project.

if you want to add more images to that Project by dragging, then it is easily done. If a folder called More Photos that contains four images is dragged onto the Photos project then this results:

Notice the new album called More Photos and the increased count of images in the Photos project. Dragging a folder to an existing project does not create a new project with the top-level folder name, just an Album. The disadvantage with this way of importing is that the Project can be huge. So the best way to import an existing library is probably to drill down to a lower level in the folder hierarchy and drag those into separate Projects, then organize the projects with Blue Folders.

On top of this, because the folders (both types) can hold Albums, Galleries, and Light Tables from anywhere, you can create these wherever they make sense. And images from one Project can be shown in any Album, Gallery or Light Table.