We present a model in which the 22 GHz H _ { 2 } O masers observed in star-forming regions occur behind shocks propagating in dense regions ( preshock density n _ { 0 } \sim 10 ^ { 6 } – 10 ^ { 8 } cm ^ { -3 } ) . We focus on high-velocity ( v _ { s } \raise 1.29 pt \hbox { $ > $ } \kern - 7.5 pt \lower 3.01 pt \hbox { $ \sim$ } 30 km s ^ { -1 } ) dissociative J shocks in which the heat of H _ { 2 } re-formation maintains a large column of \sim 300–400 K gas ; at these temperatures the chemistry drives a considerable fraction of the oxygen not in CO to form H _ { 2 } O . The H _ { 2 } O column densities , the hydrogen densities , and the warm temperatures produced by these shocks are sufficiently high to enable powerful maser action . The observed brightness temperatures ( generally \sim 10 ^ { 11 } – 10 ^ { 14 } K ) are the result of coherent velocity regions that have dimensions in the shock plane that are 10 to 100 times the shock thickness of \sim 10 ^ { 13 } cm . The masers are therefore beamed towards the observer , who typically views the shock “ edge-on ” , or perpendicular to the shock velocity ; the brightest masers are then observed with the lowest line of sight velocities with respect to the ambient gas . We present numerical and analytic studies of the dependence of the maser inversion , the resultant brightness temperature , the maser spot size and shape , the isotropic luminosity , and the maser region magnetic field on the shock parameters and the coherence path length ; the overall result is that in galactic H _ { 2 } O 22 GHz masers these observed parameters can be produced in J shocks with n _ { 0 } \sim 10 ^ { 6 } – 10 ^ { 8 } cm ^ { -3 } and v _ { s } \sim 30 – 200 km s ^ { -1 } . A number of key observables such as maser shape , brightness temperature , and global isotropic luminosity depend only on the particle flux into the shock , j = n _ { 0 } v _ { s } , rather than on n _ { 0 } and v _ { s } separately .