There are four major helmet types in use in the US Military today.  All are made from Kevlar.  The steel pot is from Vietnam and before.

PASGT	MICH	ACH	LWH	M1 Steel Pot

Personal Armor System for Ground Troops (PASGT) helmet (also called the K-Pot) is made of and has been in use since 1983. The Kevlar helmet, available in five sizes from extra small thru extra large five sizes, provides ballistic protection for the head from fragmenting munitions and handgun bullets.  Comes with a strap suspension system inside the helmet and two-point chin strap.  Can be upgraded with a four point chinstrap and pad suspension system.  Primary users: all services.

Modular Integrated Communications Helmet (MICH) is a special operations lightweight ballistic helmet that incorporates excellent ballistic and non-ballistic protection with the ability to interface with most tactical headsets and mikes used by SOF.  Standard with a pad suspension and four-point chinstrap.  Primary users: Spec Ops.

Advanced Combat Helmet (ACH) is based on the MICH design and  provides an improved helmet to soldiers, replacing the older PASGT.  It provides increased 9mm bullet protection. Comes with pad suspension system and the four-point chinstrap.  Primary users: Army.

Lightweight Helmet (LWH) was developed for the US Marines to replace the PASGT.  Looks very similar to the PASGT, but has a four-point chinstrap built-in and is about 6 ounces lighter.  Originally it had a web suspension system and being upgraded to padded suspension.  Primary users: Marines and Navy FMF.

Pad Systems from various manufacturers

Helmet Impact Testing Video

Click on images above for larger views

Personal Armor System for Ground Troops (PASGT) helmet (also called the Kevlar, K-Pot and Fritz) was first fielded to U.S. military units in the early 1980s. The helmet, available in five sizes, provides ballistic protection for the head from fragmenting munitions. It is a one piece structure composed of multiple layers of Kevlar ballistic fiber and phenolic PVB resin. For a complete story, go to olive-drab.com. This helmet is now being removed from inventory in ALL services, replaced in the Marines by the LWH (see below) and the Air Force by the Army-style ACH helmet.

Ý This is the standard helmet above

Ý This is the helmet with the kit installed (click on the photos for a much larger view)

Click here to see how to wear the helmet properly.

This another after-market comfort kit that is cheap, hot, and provides no impact protection
 

Modular Integrated Communications Helmet (MICH) is a lightweight ballistic helmet that incorporates excellent ballistic and non-ballistic protection with the ability to interface with most tactical communications headsets and microphones utilized by special operations forces. This is now integrated in to Army inventory as the MICH/ACH helmet.

http://www.globalsecurity.org/military/systems/ground/mich.htm

From NDIA Magazine Full article  

"Elder, a former U.S. Army Ranger, told National Defense that the helmet is not only comfortable and reliable, but also provides improved communications capabilities.

“First, the higher cut [of the helmet] allows a user to have complete range of motion while using the complete range of load carriage systems and body armor employed by our user groups,” he said. “It allows a user to lay in the prone and engage a target. This is huge. This task was very difficult to impossible with the old PASGT (Personnel Armor System Ground Troops) helmet while wearing most body armor and load carriage,” Elder said. "

From Natick's "The Warrior" newsletter

"What would allow the wearer to stay conscious is the innovative seven-pad suspension system. The current helmet uses a bolted-on nylon suspension with a leather headband that is fastened onto the inside headband. Many users would buy a circular pad to ease the weight stress on top of their heads.

Suspension pads consist of a comfort foam and “slow memory” foam to absorb shock. The cloth covering wicks away moisture to keep users cooler.

The MICH suspension pads are composed partly of comfort foam where the pads touch the head and mostly of “slow-memory” impact foam with the resilience of a wrestling mat.  The foam is like a shock absorber against a striking bullet.

(NOTE: In ballistic testing, while the helmets will stop a 9mm round, the impact indents the helmet approximately 1.5. inches. If the helmet were against the skull when the bullet strikes, that indentation is transferred to the skull with resultant injury and/or fracture. Holding the helmet permanently off the skull with shock-absorbing pads reduces or eliminates that scenario. Edit by OpHelm)

A black CoolMax cloth covering wicks moisture away and helps the user stay cooler. Lining the inside is a glued-on strip of Velcro fastener. Users can unhook and adjust the pads to create a custom fit. "

Advanced Combat Helmet (ACH) is based on the MICH design and  provides an improved helmet to Soldiers, replacing the older PASGT.  In addition to providing the Soldier ballistic and impact protection, it will be used as a platform to mount electronics such as communication devices, and Night Vision devices. It provides increased 9mm bullet protection. The reduced edge cut of the ACH, although reducing area of coverage, will improve the field of vision and hearing, leading to better situational awareness over the current helmet. The ACH has five basic components: The helmet shell, the pad suspension system, the retention system (chinstrap/napestrap), helmet cover, and eye strap. While the first 500,000 or so were equipped with top-of-the-line pads, the Army recently changed to a cheaper pad system that is hard and  non-conforming to the skull, resulting in pressure points and headaches. The 'GI' pads also disintegrate quickly in the rigors of combat. Some troops are removing the pads and taping rolled-up socks inside their helmet...hardly a safe procedure when a much better pad system is available and used in the past.

http://www.globalsecurity.org/military/systems/ground/ach.htm

Click here to see how to wear the helmet properly.

Army ACH complete story: Questions from a Congressman to the Army were answered as follows on May 26, 2006:

Lightweight Helmet (LWH) was developed for the US Marines to replace the PASGT. The inner 'skullcap' suspension system has now been abandoned in favor of shock-absorbing pads, following the House Armed Services Committee hearing in June 2006. Unfortunately, the pads chosen are the same bottom-tier pads the Army uses that disintegrate quickly, are hard and uncomfortable, resulting in troopers removing their helmets for comfort, even facing the ever-present and unpredictable threat of IED's, RPG's, Ambush, vehicle accidents, etc..

Ý This is the standard helmet above.

Ý This is the helmet with the kit installed (click on the photos for a much larger view)

http://www.globalsecurity.org/military/systems/ground/lwh.htm

The LWH received Milestone C approval in May 2003. Initial operational capability was scheduled for the second quarter of FY 2004 and full operational capability for the first quarter of FY 2009. Production of 200,000+ helmets for Marines started in the June 2003.  The helmet's shell is shaped like the older version, but new materials bring a 6 percent improvement in fragmentation protection and the ability to stop a direct hit from a 9mm round.  A medium-size PASGT helmet weighs 3.6 pounds as compared to medium LWH's 3.05 pounds.

M1 Steel Pot

As far as we know, the US Military no longer uses these helmets at all.  This is the helmet Doc Bob wore in Vietnam and Mark wore in Desert Storm.  They were kinda nice - you could use for cooking, bathing, etc.