The first thing to understand is that these tires are puncture resistant, but not completely puncture proof. I use these tires on four separate bicycles, two using the 28 mm width, one using the 40 mm width, and one using the 47 mm width, all of which I use for commuting and recreational rides. There is a lot of broken glass on the route to work, as well as thorns and goathead stickers seasonally. On one of the bikes wearing the 28 mm tires, I've had two flats. Once I had the misfortune to run over something, perhaps a large piece of broken glass or sharp metal, in the early morning darkness and it was sharp enough to cut (not puncture) through the tire, and the tube on the rear wheel. I didn't see the object, but it made a loud "ping" sound as it came out of the tire and bounced on the road. It left a slash more than 1/2 inch wide in the tire. Whatever this object was, I suspect any bicycle tire would have been similarly damaged, perhaps a motorcycle or car tire also. I also had another flat on the front tire of the same bicycle. While repairing this time, I noticed that the tube was a racing tube, made of very thin lightweight rubber and sized for a 20mm tire, not a 28mm. So when installed in the 28mm tire, that tube was expanded considerably larger than it should have been, and it's thin walls were stretched even thinner...no wonder a tiny thorn managed to puncture it. I ended up buying new, standard weight tubes in the correct size. One the bike with the 40 mm Marathons, I rode through a patch of grass perhaps 10 feet long along the edge of the path, and instantly picked up a number of goathead stickers. I stopped a short distance further when I saw them rotating in the tire, and pulled them out, but both front and rear tire were already punctured, and leaking slowly. I managed to ride home before both went flat, and got to spend a while putting patches on both tubes. Marathons are lighter weight and more supple than the common thick walled "puncture resistant" tubes, although not quite so light as a standard tire without the puncture resistant belt. I have had very good success with the thick tubes resisting flats, but they really change the way a bicycle rides, as the thick tube adds a lot of weight where it doesn't do any good, making the wheel very heavy. So, Marathons are not a racing tire, but a very good commuting or touring tire. My personal best commute time was on 28 mm Marathons (8 miles in 28:45)...on a 35 year old 12 speed lightweight bicycle. If you want to go faster, Schwalbe makes other tires with lighter, more exotic and more expensive materials under the tread (there is a Marathon Racer, and a Kojack slick, among others). And it's not completely invulnerable, but very tough nonetheless. The "Greenguard" belt appears to be merely an extra thickness of rubber under the tread area, and helps prevent punctures simply by virtue of it's extra thickness. It's offers the benefits of a thick tube under the tread where you need it, without the weight of extra rubber on the sidewalls and in the area of the spoke nipples. It doesn't appear to be particularly difficult for a sharp thorn to penetrate, and if the thorn is long enough, it can reach your tube. The "Greenguard" belt isn't kevlar, although Schwalbe offers a kevlar belt in the Marathon Plus, their more expensive tire. However, the extra puncture resistance of the Marathon Plus does come at a cost, that tire weighs more and doesn't roll quite so well as this tire. So, you decide how much puncture protection you need, and choose the tire which seems to fit your needs best. One thing to be aware of is that the belt adds slightly, 3 to 5 mm to the height of the tire, so be sure you have clearance at your front fork and at your rear brake bridge. If you don't have significant clearance with your existing tires, consider downsizing the width when using Marathons. In addition to reducing the instances of flats from punctures, these tires have a reflective band which is highly visible in the lights of cars. I've had coworkers tell me that they could see me several hundred feet away in the darkness, lit only by their own headlights. I do have battery powered lights as well, but the first thing they reported seeing was two bright white reflective circles moving in the darkness. This feature added to the reduced frequency of punctures makes these my favorite commuting tire.
