Like I said before, this is a very under-used feature, so it’s been around for a while. I know for a fact that Layer masking is available in CS1+ but anything earlier, I’m not sure. (hopefully you all can help us out with that in the comments) By creating a path on the very top-most sublayer of each Layer, you can use that path as a mask by toggling the Mask Button in the Layers Pallete. If you hit the masking button without having your desired path on the top, Illustrator will automatically use whatever path is in the top-most sublayer (or shape layer). So it’s very important to make sure the shape you wish to use as a mask is on top of everything else in that layer. Using Layer Masks will save you time by avoiding tedious un-grouping, grouping, releasing, and applying clipping masks with the ease of a simple toggle button. Attention the hierarchy of your layers and sub/shape layers is essential in order to get the most out of this feature, but once you get the hang of it, it should definitely speed up your work flow! You can create a unique layer mask for each and every layer in your Illustrator document. I often find that a layer mask that matches my artboard can come in really handy. So I’ll be using that as an example. My artboard, in this case, is 468 X 350 pixels. To create a mask you can use a variety of tools (any tool that creates a path). In my case I’ll be using the Rectangle tool. In order to easily make a rectangle at the exact size of your artboard, simply click once on the artboard without dragging. You will then be able to type in the exact dimensions. Click OK and you will have your rectangle path. (fill and stroke don’t matter) Now that you have your rectangle, you probably want to align it perfectly to the artboard. You can do this easily with the Align palette. (Window >, Align) With your shape selected, align it horizontally and vertically to the artboard by clicking on these two buttons in the Align palette: (make sure that “Align to Artboard” is enabled). If you haven’t already, make sure your path is the top-most path in your layer. [edit: Select the LAYER, not the path,] then click on the Mask toggle button in your Layers palette. With a single click in the Layers palette, you can now toggle the visiblity of everything in that layer! You just have to make sure that your desired masking path REMAINS ON TOP or “FRONT” of that layer. Now that you’ve mastered the art of layer masking, use it to your advantage. You can freely move and modify all of your shapes in the layer without releasing the layer mask! This not only saves a lot of time, but gives you a better visualization of what a finished and/or “cropped” file will look like but with much more workflow freedom than applying individual clipping masks to groups of shapes. You can always toggle the layer mask off with a single click if you need to see everything. Quick Tip: I like to “lock” my layer mask in the layers palette so I can’t ever select or move it’s position. Source.