Your Pectoral muscle group is made up of two muscles, the pectoralis major and pectoralis minor. Both of these muscles cover the front of the chest and connect the front wall of the chest to the upper arm and shoulder blade. These muscles are important for pushing and reaching the arm across the body.
These muscles tend to become very tight in individuals who have poor posture, have a desk job, or perform excessive weight training without also stretching. Tight pectoral muscles can lead to rounded shoulders , decreased ability to raise your arm overhead and shortening of the muscles in the front of your neck.

Try the following stretch to open up your chest and decrease pectoral tightness:
Doorway Stretch: Stand in a doorway, put one hand on each side and position elbows at approximately shoulder height. Keep your trunk upright as you step into the doorway with one foot. You should feel a strong but comfortable pull in the front of your chest. Hold 30 seconds, repeat 2-4 times. Stop if this causes shoulder pain.

Photo courtesy of MedBridge