What Does Modern Science Say About Acupuncture ?
Thanks to Magnetic Resonance Imaging ( MRI Scanners ) and other medical technologies scientists have been able to detect and identify  many of the mechanisms involved with Acupuncture therapy.  
Research has indicated that many of the effects of acupuncture are due to the release of various chemicals in the brain, neurotransmitters and hormones. This has been observed repeatedly using MRi Scanners by researchers across the world.

Hormones and chemical messengers produced in the brain control almost every aspect of our existence. Acupuncture has been seen to   effect changes in the central nervous system for example.  The release of Endorphins are said to be over two hundred times more potent than morphine. The benefits of Acupuncture for pain control are well documented and researched.

Serotonin is another chemical that has been identified. Serotonin is said to affect our moods,  which is why acupuncture is said to help with a range of depression and anxiety disorders.

Other observations include positive changes to the circulatory system including decreased blood pressure and the immune system where the T-Cell count had increased.  These are just a few but important examples.

How does it actually work?
Although its effects can now be observed using various scientific instruments and of course the testimony of millions of  patients over thousands of years it is still not fully understood from a western point of view. What is known is that inserting fine needles at very specific points in the body stimulates various sensory receptors. These receptors then transmit signals to the hypothalamic-pituitary system at the base of the brain.

From a Chinese medicine point of view the explanation is similar but more takes into consideration the view that the human body also has a series of energy channels called Meridians. These meridians are not some imaginary lines but are often over main arteries, veins, nerves and major organs. Points on the meridians can be used to affect changes at all levels, physical, mental and emotional.