They promise so much. They'll get us fit, we'll reach for new exercise goals, and we might lose weight on the way.
For many that happens, but for some, fitness trackers and calorie counters can become an obsession - and have life-threatening consequences.
We investigate how these technologies and the rise of food bloggers and social media are seriously affecting people with eating disorders.
In this investigation we've taken the decision not to mention the weights of anyone we've featured. There are a number of reasons for that.
Experts have told us that anorexia can be a hugely competitive illness, and it could be damaging for those watching with eating disorders.
It's also worth mentioning that weight is just one of the factors in this hugely complex illness.
Finally, many of those we spoke to took a long time to decide to speak out, it took courage to talk about it, and we felt we had a duty and responsibility to those who bravely shared their story.
Posting what we eat has become second nature. But for people with eating disorders that can give them the validation they're looking for.
Health experts and charities have warned people with eating disorders to "do anything you can to avoid" calorie counting apps.
Weight loss apps and devices such as Fitbits can be dangerous for those with eating disorders, experts have warned.