Canine obesity, as with human obesity, can be caused by health problems but it mostly simply due to energy intake being greater than energy expenditure, meaning that the body is taking in more than it needs.
Dogs have the benefit that we mostly control their food intake so the problem should be easier to control.
The first step for any pet which is new to your house or has suddenly put on weight should be a vet visit. If your vet confirms there is no underlying health reasons (such as thyroid problems) then you can begin a slow and steady diet.
We will shortly be updating this article with help on how to put your dog on a diet, gently increase the exercise appropriate to your dog and give you some insights into different foods and treats and which may be good (or not) for your pooch.