I understand how to hide the poly what is the key command to unhide them?
also how do u invert the hidden selection.
To hide a polygroup use CTRL+SHIFT and click onto the group you want to remain visible.
To reveal the hidden parts CRTL+Shift and click on the canvas away from the model.
You can hide portions of the model based on a drag rectangle. Use CTRL+SHIFT and left mouse button to drag out a GREEN rectangle but release the SHIFT key and the rectangle goes RED. Then release the mouse button and everything inside will be hidden.
To reverse the mesh visibility, drag a small rectangle anywhere on the blank canvas (without selecting any part of the model)
Paraphrased from pages 298-299 of the Practical Guide.There’s no ‘invert visibility’, (although there is the ability to inverse masking selections). To visibly isolate a small group, say a single polygon in size, use the method described above to hide part of the model (Ctrl+Shift+drag > release Ctrl+Shift > release mouse button). Then Ctrl+Shift+click on groups you want hidden. That way you can isolate even a single polygon, or a collection of groups. This is dead handy for cases where a part of your model is made of several groups. For instance, the way I make eyes, they usually consist of 3 groups (pupil, iris and eyeball). This method is the only reliable way of isolating the eyes so that I can mask them whilst I model eyelids etc.
Hey Moochie,
To invert visible and invisible polygroups, press control-shift, click and drag outside the object, stay clicked and release control-shift (the rectangle turns red), then release the mouse button.
Another handy trick to make selective multiple polygroups invisible… Control-shift, click on the first polygroup to make invisible (an eye for instance which becomes the only visible polygroup). Keep holding down Contro-shift and click the visible polygroup (the eye) which inverts visible polygroup and invisible polygroups. Continue to click on each visible polygroup you want to make invisible (the other eye for instance) leaving the group(s) you want to work on visible.
Sven
Well, I’ll be jiggered! You are totally correct. Thank you … another handy technique learned. 
If you click the Help in ZBrush, then click on ZBrush Essentials, at the end of the list you’ll see that there’s a Hotkeys & Mouse Actions PDF. Very handy to print out. 
Thanks Marcus … I hadn’t seen that list before. I’ve got a two page list of hot keys propped up against the wall next to my monitor, but it’s not as useful as the one you’ve pointed out. The ‘c’ and ‘v’ hot keys look particularly useful!
Moochie,
Now that I look at the hotkey list I see that reversing the visible/invisible polygroups of a model only requires a control-shift and click and drag outside the model. For some reason I’d conditioned myself to release the control-shift before releasing the mouse button. Works both ways. 
Another little tip to remember - you can undo (Ctrl-Z) an errant control-shift click… meaning you don’t have to abandon then redo the whole selection process from scratch if you accidently hide the wrong polygroup.
(Marcus, thanks for pointing to the list - I’d lost mine in the morass of paper on my desk and forgotten where I printed it from!)
Sven