I have been looking at getting a German Shepard puppy and wanted to know if anyone has any advice on introducing it into the house that already has a 13 year old cat.

I've done it 3 times at least.2 dogs were gsds too . My advice would be too make sure the cat has places he can go that puppy cant get to remembering that cats think vertically as well as floor level so high places. .
I have a baby gate across one room permanently so the cat can eat in peace and use her litter tray without disturbance
.previously the cats ate on a workspace but she is too old now to do that .
As for pup it is your responsibility to train puppy to leave the cat alone especiallyuntil they are happy around each other. Remember cats will protect themselves and claws are sharp . .  Cat shouldn't have to change its behaviour if at all possible pup has to learn how to behaviour around the cat . Might be worth introducing them when pup is on a short lead and cat is in tje roo.
The dog I have now is 10 and mostly leaves the cat alone which is what my cat wants .not all do . I do have to occasionally step in if dog tries to chase though.
It sorts itself out and is great starter for training pup anyway to be calm and get impulse control

Thanks that is useful information and also reassuring that it is possible.

Spot on furriefriends.  Yesterday we introduced a new dog (not a puppy but an adult street dog) to our house and our ancient 20 year old cat has happily taken herself upstairs where she is even allowed in the bedrooms.  She will reintegrate when she is ready.

I always say cats rule ! Dogs have to learn how to live alongside . Think its pretty much the same with cats and humans .cat comes first

Thanks for all of your posts, it has been really helpful in deciding about the dog.