Cosmic rays interact with mesoscale configurations of the interplanetary magnetic field. We present a technique for calculating such configurations in the inner solar system, which are due to streams and source conditions near the sun, and we construct maps of ‖B‖ for some plausible stream and source conditions. One effects of these mesoscale configurations on galactic cosmic rays is shown to be an out of the ecliptic gradient drift sufficient to explain Forbush decreases. The effects on solar energetic particles include small polar drifts due to the field gradients and a possibly large modification of the time-intensity profiles and anisotropy characteristics due to the formation of mirror configurations in space. If a diffusion model is applicable to solar particles, the true diffusion coefficient will be masked by the effects of streams. A conceptual model which incorporates these ideas and those of several other models is presented.