Could somebody explain me the difference between them? They really appear to have a similar usage and meaning. I feel that と共に can be used instead of と一緒に in formal writing , since they both mean "along with", and besides, they both express the idea that two things are changing at the same pace. However, にともない seem to have only the meaning of two things changing together. Actually, it seems to be similar につれて and に従って, but maybe I'm wrong.