We argue that parallel propagating Alfven waves steepen to second order in the wave amplitude, rather than first order, as is the case for oblique MHD waves. We derive an analytical solution for the structure of weak switch-on shocks. The Rankine-Hugoniot relations for finite amplitude switch-on shocks are obtained in a compact form. The differential equation for resistive switch-on shocks is solved, and the resistive critical Mach number is obtained. The differential equation for resistive switch-on shocks with finite ion inertia is identical to the one obtained in the pure resistive case. We argue that efficient electron heat conduction can significantly extend the range of upstream parameters for which switch-on shocks are expected. ¿ American Geophysical Union 1988