The Magellanic Stream and several high velocity clouds have now been detected in optical line emission .
The observed emission measures and kinematics are most plausibly explained by photoionization due to hot , young stars in the Galactic disk .
The highly favorable orientation of the Stream allows an unambiguous determination of the fraction of ionizing photons f _ { esc } which escape the disk .
We have modelled the production and transport of ionizing photons through an opaque interstellar medium .
Normalization to the Stream detections requires { f _ { esc } } \approx 6 \% , in reasonable agreement with the flux required to ionize the Reynolds layer .
Neither shock heating nor emission within a hot Galactic corona can be important in producing the observed { H } \alpha emission .
If such a large escape fraction is typical of L _ { * } galaxies , star-forming systems dominate the extragalactic ionizing background .
Within the context of this model , both the three-dimensional orientation of the Stream and the distances to high-velocity clouds can be determined by sensitive { H } \alpha observations .