Flat feet, fallen arches, low arches, overpronation, pes planus,and hindfoot valgus are all terms which describe a lowering of the arch of the foot and a tilt of the heel towrds the midline.  

None of these terms are a diagnosis,and the term overpronation has been and continues to be a topic of great debate over the last decade in the medical profession.  

Pronation is a medical term used to describe a normal movement pattern of the foot that occurs during walking to absorb shock and adapt to different walking surfaces.

Reserach shows that there is such a large variation in arch height and foot shape in the general population, and many people have flat feet (and it causes them no problems at all).  This makes it is very dificult to define what "normal" arch height is, and so impossible to define what "over" pronation would be, hence the debate about overpronation continues!

While the academics worry about how to precisely measure and define what normal is (no easy task!), most clinicians are now working on a tissue stress model theory which looks more at how the foot moves during walking/running, and what structures and soft tissues may be overloaded leading to injury.

Many conditions have been linked to having flat feet:

Plantarfaciitis

Bunions

Morton's Neuroma

Achilles Tendonopathy

Baxter's Nerve Entrapment

Tarsal Tunnel Syndrome

Medial Tibial Stress Syndrome 

Posterior Tibial Tendon Dysfunction

Knee Pain

Lower Back Pain