  We present the implementation and the first results of cosmic ray (CR)
feedback in the Feedback In Realistic Environments (FIRE) simulations. We
investigate CR feedback in non-cosmological simulations of dwarf, sub-$L\star$
starburst, and $L\star$ galaxies with different propagation models, including
advection, isotropic and anisotropic diffusion, and streaming along field lines
with different transport coefficients. We simulate CR diffusion and streaming
simultaneously in galaxies with high resolution, using a two moment method. We
forward-model and compare to observations of $\gamma$-ray emission from nearby
and starburst galaxies. We reproduce the $\gamma$-ray observations of dwarf and
$L\star$ galaxies with constant isotropic diffusion coefficient $\kappa \sim
3\times 10^{29}\,{\rm cm^{2}\,s^{-1}}$. Advection-only and streaming-only
models produce order-of-magnitude too large $\gamma$-ray luminosities in dwarf
and $L\star$ galaxies. We show that in models that match the $\gamma$-ray
observations, most CRs escape low-gas-density galaxies (e.g.\ dwarfs) before
significant collisional losses, while starburst galaxies are CR proton
calorimeters. While adiabatic losses can be significant, they occur only after
CRs escape galaxies, so they are only of secondary importance for $\gamma$-ray
emissivities. Models where CRs are ``trapped'' in the star-forming disk have
lower star formation efficiency, but these models are ruled out by $\gamma$-ray
observations. For models with constant $\kappa$ that match the $\gamma$-ray
observations, CRs form extended halos with scale heights of several kpc to
several tens of kpc.
