Exposure cracking, sometimes referred to as sun-checking or weather checking, is a term used to describe the behavior of stretched rubber when exposed to the atmosphere under conditions which cause cracking. Cracking can occur when a rubber product under slight tension is exposed to the atmosphere or to ozone. It can occur whether the product is at rest or in motion, in sunlight or in the shade ; or whether the exposure is in the daytime or at night. Susceptibility to cracking may be different under these different conditions. In a recent very excellent and comprehensive paper by Newton, many known facts about weathering and cracking were assembled, and the theory was proposed that all cracking of stretched rubber under exposure is due to ozone. A careful distinction was drawn between this and the crazing or weathering which occurs on unstretched rubber. If, as he contends, ozone is the active agent, the term “sun cracking” is obviously incorrect, and the term “exposure cracking” is to be preferred in describing the behavior of stretched rubber when exposed to atmospheric conditions or ozone. In spite of the fact that we now consider all cracking of this type to be due to ozone, the testing of compounds for resistance to cracking is not a simple matter. The more obvious laboratory test methods disagree frequently with each other and with service behavior. It is difficult offhand to explain why a stock containing wax, which is excellent under static exposure of test strips, may be very bad as a tire sidewall ; why an inflated tire stored in the shade may crack worse than a similar tire stored in the sun ; why a tire on a parked car may crack worse than a running tire ; or why a good compound in resisting sun exposure may appear to be inferior in resisting ozone. The present work adds further weight to the new well-established ozone theory. It demonstrates clearly, however, that neither outdoor nor indoor testing is a simple matter. This work also offers for consideration new testing methods and some explanation of their meaning.