I get this question a lot, and rightfully so. It is really important, but like most things in Chinese medicine the answer is "it depends". This is because acupuncture is a individual based system, not a disease based systems. We don't treat diseases we treat humans. So instead of treating headaches, we treat your headache, which is different than your cousins headache, which is different than your soccer coaches headache. This can be really difficult to explain and understand in a world where diseases are the focus. If the M.D. diagnoses you with migraine headaches, you most likely will receive the same treatment as everyone else with migraines, this is because they treat the disease and not the individual. Treating the disease can be very handy in emergency situations where there isn't time to divulge in the details of the individual, but isn't very effective for long term solutions. This is where Chinese medicine excels.
So... what does Chinese medicine treat? I will answer it this way: our job as acupuncturists is to stimulate the bodies own natural resources or healing abilities. Therefore what acupuncture treats is really only limited by the bodies own healing resources, and that is different for every individual. Whats awesome is that our bodies have this innate desire to be healthy and a huge capacity for healing, and acupuncture is able to tap into this and remind our bodies how to heal.
This means, theoretically Chinese medicine can treat any disease, but not necessarily every person. Different people can have different capacities for healing. It is also on the patient to improve their chances of healing, by eliminating barriers that block their body from healing (i.e. diet and lifestyle). This is hard for people to understand, because our brains like lists, and definitive answers, yes and no... welcome to Chinese medicine. It is a completely different way of thinking. If you have a specific question about a problem you are suffering from, even if you have never heard that acupuncture can treat it, bring it to your acupuncturist.
Happiness can be found, even in the darkness of times, if one remembers to turn on the light. - Albus Dumbledore