My dog, an 18mth old Jack Russel cross trembles whenever I hold him and even trembles whilst asleep on his back with all four limbs twitching. Is this behaviour harmfull to him?
What does it mean? Do I just accept it as the way he is? He Is a very intelligent and lively dog who views life as one big adventure! Inspirational!

It never hurts to discuss things like that with the vet when you are in. In some cases, I think small terrier do tremble like that.

Terriers and game dogs that aren't allowed to do what they were bred to do (hunt), seem to tremble like that more than any other type of dog. You have a high energy little dog there. While you are holding him, it is most likely he is trembling due to excitement. Make careful note of what the possible triggers are when this behavior occurs. Are there birds, cats, any other animals around outside in the yard or street? You may not notice them, but a dog's hearing/senses are much more acute than ours. Even my big terrier twitches when she senses something exciting going on outside that I can't see or hear. Usually, after a while, I realize it is a squirrel or bird that is triggering her behavior. Regarding the twitching when he sleeps, perfectly normal and usual behavior for a lot of dogs while sleeping. Happens more frequently when they are puppies and tapers off as they age, but some dogs will do that their entire lives. If you are concerned, a discussion with your vet never hurts.