Did you know...

Helmets have been around since well, the dark ages. Leather or metal have protected combatants over the ages. Knights had them, Pikemen had them, Roman legionnaires had them, as did the Greeks. Helmets were used for protection and status. 

During the Revolutionary War the British had them, though they weren't very combat functional. For the most part, in the modern era, 1900's, Military outfits still did not "issue" helmets to their men. The Germans marched into the trenches in WWI wearing the "Pickelhaube", the spiked helmet. This was made of leather and offered no protection against shrapnel.

Here it comes...

Did you know, that because of the amount of head wounds sustained by the troops in WWI, that the Militaries officially began issuing helmets. 

                              The First U.S. helmet was the M1917 based on the English design. This helmet was worn by the U.S. Military branches until it was replaced in 1941 by the M1 Combat Helmet. 

The M1 combat helmet was in service for many years, it became the iconic American helmet. This helmet saw service in both theatres of WWII, it marched into Korea, Helicoptered into Landing zones in Vietnam. The M1 had a few variations such as the M1C and M2. As a matter of fact, the M1 helmet liners are still in use today by Navy Seals during training. Over the years improvements were made, chinstrap design, etc. The M1 was finally phased out during the mid 1980's by the PASGT Helmet and Vest combo.

M1

The PASGT (Personnel Armor System for Ground Troops), This helmet was used by the Military until it was phased out by, in the Marine Corps in 2005, LWH, the Lightweight Helmet. and the Army replaced them in 2003 with the (ACH) Advanced Combat Helmet.

PASGT

Army (ACH)

Marines LWH

Currently testing is going on for a replacement helmet for both the LWH and the ACH. The new Helmet is the (ECH) Enhanced Combat Helmet. This will use Thermoplastics instead of Ballistic fibers and if adopted will be used by the Army, Navy, and Marines.  The helmet will be close in design to its predecessors but will be thicker and offer 35% more protection against small arms and fragmentation.

As for collectors, the helmet that carries the most value of U.S. Helmets, isn't the oldest (M1917) nor any of the newer designs. The Helmet getting the big bucks is still the icon, the M1. Which Vintage M1? The Vietnam era, especially if it carries any authentic Vietnam era insignia on it.