A "particle" of charge Ze moves with velocity v, mass M, energy [formula], through a "substance" of atomic number Zo, density N atoms/cm3. We have to consider the process of slowing down, and the nature of the trail of ions produced. Very slow particles are of little interest, as their range is very short. If Z is large, or if the particle is an electron, there are complications which we postpone. Thus, our considerations will apply particularly to protons, deuterons, o< -particles, and mesons.