If you look at an electromagnetic spectrum you will see that gamma radiation has the shortest wavelength and highest frequency therefore, the highest energy. This means that these rays are very penetrating and can go through most substances. In fact, it takes several centimeters of lead (a very dense material) to stop them. As they pass through materials, the rays interact with the substance. If the the substance is made out of living cells, these rays can damage the cells and can lead to mutations known as cancer.