Dog-dog aggression is unfortunately very common but it actually responds very well to treatment, especially when addressed early in its development. However, as with all behaviour problems, accurate diagnosis of the motivations for the aggression is essential for appropriate treatment to be devised.The main causes of aggression towards unfamiliar dogs for example that are met on walks are social incompetence, fear, pain and inter-male competition. Although usually only an influencing factor, the effects of the owner's behaviour can also initiate problems between dogs.Social competence, i.e. interacting appropriately and confidently with other dogs, starts to develop at a very young age. Puppies in their socialisation period must meet a range of other dogs, including dogs of different ages, gender, breed, size and body conformation. Learning about social behaviour continues into maturity, therefore it is very important that dogs are allowed to interact with other dogs on a regular basis throughout life in order for these social skills to be maintained. A dog can only truly learn how to act appropriately around other dogs if he can interact with them off lead as well as on lead, therefore owners must teach their dog good recall in order to allow off-lead interactions. If a dog does not understand how to interact with others appropriately then fights can ensue. For example, a dog that learnt as a puppy that other dogs are fun but never learnt to curb his enthusiasm for play or to read other dogs distancing signals (appeasement or aggression) may be attacked if it continues to "hassle" the other dog. This can lead to fear in the under-socialised dog as he learns that interactions with other dogs are negative. Dogs that were inadequately socialised with other dogs as puppies may grow up lacking confidence interacting with other dogs as they are not used to being around them and do not know how to communicate either. Anything not experienced during the critical socialisation period is very likely to be met with fear later in life. However, even a well-socialised dog can become fearful of other dogs if they are attacked. Older dogs who are suffering from the pain of arthritis, or any other dog that may be experiencing pain or illness, can also associate interacting with another dog with a negative feeling. For example an older dog with arthritis may associate another dog jumping on it or bumping it playfully may learn that interactions with other dogs are painful and therefore to be avoided. Dogs have several options when dealing with a negative situation. They can run away, hide, freeze, or use appeasement or aggression in order to make the negative stimulus, in this case another dog, move away. Which option the dog chooses can be influenced by genetic traits, previous learning experiences and the individual circumstances of the situation. For example a genetically fearful dog might be more likely to develop fears of other dogs, but within this fearful group of dogs, the very fearful might be more likely to run away or hide, whereas the more confident of the fearful dogs might be more inclined to use aggression. However, if the dog is on a lead or backed into a corner he may not be able to use a preferred coping strategy of running away or hiding, and freezing may not deter an attack. He might try to use appeasement behaviours but if the threatening dog is not well socialised it may not recognise and respond to these calming signals and continue its threatening behaviour, whether meant or perceived. If the appeasement does fail then he will have no option but to use aggression. Each dog will learn from these experiences, which will then influence the way they behave in future encounters. For example, a dog who was previously scared by an interaction with another dog may have learnt that running away was not successful, perhaps because it was on a lead, and appeasement behaviours were ignored. So rather than wasting time with these behaviours he will go straight for aggression in subsequent situations in which he feels threatened. The owner's behaviour will also influence their dog's behaviour. One example of this is if the owner is worried about how their dog will react around another dog. If this owner sees another dog they may call their dog back and put it in the lead which they then hold very taught in order to prevent their dog attacking the other dog. The dog will not realise that his owner is worried about his behaviour but may inappropriately associate their owner's behaviour with the other dog, thinking that the owner is also frightened of it. This confirms to the dog that other dogs are something to be worried about and reinforces his behaviour. If the dog does react to the other dog and the owner uses punishment to try to stop the behaviour then the dog is likely to become more scared. Not only does he have to worry about the other dog but he also has to worry about his owner's threatening behaviour. This will escalate the dog's arousal and fear and will likely exacerbate any aggressive behaviour which will probably become worse over time as a result.Another motivation for aggression is inter-male competition. This is more likely to occur in entire (uncastrated) males, however, if this behaviour becomes well established as a learnt behaviour then castration may not have any effect. Especially if fear has become a component of the behaviour, in which case the aggression might become worse due to a loss of confidence (testosterone provides dogs with confidence). Although it might appear easy to diagnose inter-male aggression - the male dog only shows aggression to other male dogs - it might in fact be fear aggression that is specific to male dogs. For example a male dog might have been attacked by another male dog in the past and associates male dogs with something bad happening but not female dogs, which they continue to enjoy interacting with. Therefore, accurate diagnosis through careful history taking is necessary before an appropriate treatment plan can be devised.