Plantar Fasciitis is the most common cause of foot pain.  It is a result of a constant strain on the plantar fascia on the bottom of the foot where it attaches to the heel of the foot.  It is mostly caused be standing too much and improper footwear.

Severe pain is usually felt on the bottom of the feet first thing in the morning.  The pain usually decreases after a few minutes of walking around, and is then worse at the end of the day.  It improves with long periods of resting from precipitating activity.