My pup is nearly five months old and will bark furiosly at other dogs that we pass on walking. She only does this when on her lead and she can't reach them. If we are in the park and she is free she just goes up to every dog and wants to play. This behaviour on lead  can sometimes provoke a nasty response from the other dogs as I presume they think of her as a threat and the owners too who presume that she is not friendly. At the moment I'm only hanging on like grim death to her lead as she lunges towards them and saying No which seems to have no effect at all. She gets so wound up that she doesn't seem to hear me. Any suggestions would be appreciated. 

Hi Gaby,
I too have a GSD (almost 11 months now). He doesn't bark at other dogs on lead, but used to try anything to try and get to them and play. I have found that getting him to sit by the path in the park and watch the other dogs go by is helping. If he goes to move I ask him to sit and praise and reward for the behaviour I want. He's now much better and when walking I ask him to 'watch me' and praise for that, or if it's all a bit much just stop and get him to sit and calm down. I'm sure most other dog owners understand.
Try getting her to sit at some distance from the distractions and gradually move closer day by day, praising/rewarding the correct behaviour. If she starts barking again move back a couple of steps. I'm not sure this approach is best if she is scared when on lead, but if you're sure its just excitement/frustration it's certainly helping my GSD :) He will even come back and sit and wait to be released to go play 80% of the time (still working on the 20%)
Good luck
:)

Why should she be scared when on her lead? I would have thought that it would make her feel more secure. I doubt very much that she has any fear, she does not seem to be frightened of anything. Quite the oposite. I wish she would be more cautious of approaching other dogs and people as they are not all friendly. At the moment she thinks everyone wants to play.

I will try the sitting by the path and see if this works. As I said in my post she turns a deaf ear when other dogs are in the vicinity but willing to try anything. Thanks a lot.

Sometimes dogs can be more nervous on lead as they are unable to run or even show the kind of body language they would like to to show they are no threat, it does sound like she is more frustrated though.
As for the deaf ear, you need to make sure you are far enough away from distractions at first so that she can and will listen to you. Increase the distance slowly. I have also found doing some 'control' exercises at home helps to. Waiting at the back door until released etc. I also ask him to sit then put a treat down in front of him, if he moves towards it (at first I am half covering it with my hand) I take it away. If he stays still I click and pick up the treat and give it to him. Now I'm working more on eye contact as I wish to pursue obedience etc. It may help her understand that she needs to exert a little more self control.
HTH
:)

Hi gaby, dogs are somtimes more worried when on leads as they cant use the defence of running away and the lumps on the lead end ( us) often interfere with the meeting/greeting rituals withut realising it, so poor Fido thinks hes in for it because he cant use the polite greeting form. Some dogs are on leads for the good reason they may go for other dogs. The suggestion of sitting by the path is a good one which worked for us at the same stage youre at, we also went to a class for socialisation to take the novelty out of meeting other dogs and used a long line to stop him bolting towards everyone like long lost friends, so now he knows to drop and wait for me before approaching strangers. So if you want her to be more restrained, train it. :)

All the GSD pups I know bark - excited they bark, see something new, they bark, see a friend, they bark seems to be one of their things and they also appear to grow out of it.

Just back from the park. Your advice works from a distance (a long way off). I can see this taking quite some time LOL

We go to the park twice a day as I only have a small back yard and she needs to stretch her legs. On these walks she meets loads of other dogs and today met a staffy. I spotted it from a distance but round a corner and so was able to get my pup to sit before she spotted it. This meant at least she was calm when the dog approached (also on a lead). She did not bark but immediately used the dominant pose of putting her neck over the neck of the Staffy. The Staffy growled and my pup growled meanwhile the owner and I were chatting whilst keeping an eye on our pets. The owner of the other dog then decided to go. My pups playmate from 8 weeks old, (twice a week) with my daughters adult doberman x german shepherd. They are always under constant supervision but my daughters dog has regarded her as her own pup and will not put her in her place when she gets too boisterous. Needless to say my pup takes advantage of this and this might have given her the idea that she is invincable and always top dog. I can see one day this will get her into trouble. 

Dogs bark for many reasons, one is frustration, your dog, when on the lead, is excited by the arrival of other dogs and will bark from excitement and then frustration if she cannot reach them.

Allowing your dog to approach all others could end in disaster for either your dog or another at some point in the future, especially if yours is off lead and the other is on lead, as mentioned by someone else, a dog that is "constrained" will be unable to "choose" flight or fight etc; also it is really rude to allow your dog to invade another's space and make it feel uncomfortable in this way and the reaction may well be a "chomp" from dogs who do not appreciate your dog's advances.

It is a must to allow socialisation with all sorts of dogs but do make sure that YOU choose if and when that happens and that those she mixes with are appropriate eg are well socialised themselves and of "equal" standing eg I would not allow a big boisterous puppy free rein with a smaller, more reserved breed necessarily.

Puppies need to learn how to approach other dogs, recognise when it is safe and when it is not and how to avoid trouble if necessary etc etc.

I never allow my pup to aannoy others. When I see a dog approaching I always put her back on her lead and only allow her off if the owner of the other dog says that it is alright to do so. I am quite the opposite to what you seem to think. It is only the last week or so that I have allowed her off lead atall. Preferring to use her extending lead. If I remember rightly itwas you who encouraged me to do so.

"If we are in the park and she is free she just goes up to every dog and wants to play."

"I wish she would be more cautious of approaching other dogs and people as they are not all friendly. At the moment she thinks everyone wants to play."

Obviously I misunderstood your posts.

Sorry! shlould have said when given permission. I was trying to illustrate that she is generally friendly to all, man and beast.

Gaby - I'm coming in a bit late on this but just wanted to tell you that you're not alone. My Great Dane pup, who is going to be 7 months tomorrow, also acts like yours - going beserk when she sees other dogs and thinking everyone wants to play with her and being very boisterous (climbing over them, wrestling them to the ground, etc) when she is allowed near them - and so getting attacked by other dogs a lot. Like you, I think we inadvertently contributed to her rough play because when we took her to puppy class, it happened to be made up of much smaller breeds and as a Great Dane, she towered over everyone and had a big physical advantage and so I think she learnt to be a bit of a bully. Although, having said that, everyone tells me that it is just normal puppy play behaviour and just looks a lot worse on bigger dogs. She doesn't bark but the lunging was terrible, particularly as she is HUGE (45kg already) and I was really losing control over her. I have tried various things, including getting her to sit and remain calm when we see a dog in the distance, and also practising approaching "stooge" dogs in Obedience class and making her "Leave" them until I allow her to say hello (using either Halti or choke chain to maintain control) - she is still very excitable when she meets other dogs but she is SLOWLY getting better! I also found an off-leash park where the owners are all quite relaxed and the dogs all run around freely and play with each other - so there isn't such a problem with one dog being on leash and the other off it. I find that size is a big factor and at this park, Honey can find bigger breeds who can play rough with her - and then everything is OK. And I think it is helping because it gives her an opportunity to play and learn canine social skills and also take the novelty out of other dogs. But I don't let her off immediately - I make her walk nicely beside me for a bit, WITH the distraction of other dogs, before she is allowed off. I also ask other owners first if they are happy with me letting her off - even though I know I have as much right as them to let her off - but it just makes me feel better.
The other big thing is to tell other owners straight off that your dog is only 5 months. I find the bigger breeds are at a real disadvantage here because their size means people expect them to behave like adults and don't make excuses for otherwise puppy-ish behaviour. I have this problem with my Great Dane puppy who at 5 months was already bigger than a fully-grown GSD...so I find that letting people know immediately that she is only a puppy helps them excuse her behaviour rather than think she is a vicious dog.
Good luck!
Hsin-Yi
(ps. have a look at my post about "Dominant...or just playful?")

Forgot to add, Gaby, that your tension/worry about other dogs maybe transmitting itself through her leash when she is on lead, which may be why she acts worse when on leash. I know I do! I get quite anxious when I see another dog approaching and start to brace myself - and although I know it's bad and I'm working on staying relaxed, I'm sure my tension is transmitting itself to my dog.