We discuss the physics of the transport of energetic charged particles in the heliosphere. We put this in the context of using fast charged particles as probes of the heliospheric magnetic field. Since these particles move rapidly in the electromagnetic fields within space, they provide information about the global nature of the field itself. Although the magnetic field can be observed at a point in time and space using a magnetometer, the global field is not easily determined. This is where the information from energetic-particle observations, combined with analytic theory and modeling, can be valuable.