  Gamma-ray emission produced by interactions between cosmic rays (CRs) and
interstellar gas traces the product of their densities throughout the Milky
Way. The outer Galaxy is a privileged target of investigation to separate
interstellar structures seen along the line of sight. Recent observations by
the Fermi Large Area Telescope (LAT) shed light on open questions of the EGRET
era about the distribution of CR densities and the census of the interstellar
medium. The gradient of gamma-ray emissivities measured in the outer Galaxy is
significantly flatter than predictions from widely used CR propagation models
given the rapid decline of putative CR sources beyond the solar circle. Large
propagation volumes, with halo heights up to 20 kpc, or a flat CR source
distribution are required to match the data. Other viable possibilities include
non-uniform CR diffusion properties or more gas than accounted for by the
radio/mm-wave data. Gamma-ray data constrain the evolution of the
Xco=N(H2)/W(CO) ratio within a few kpc from the Sun. There is a significant
increase by a factor 2 from nearby clouds in the Gould Belt to the local spur.
No further significant variations are measured from the local spur to the
Perseus spiral arm. At the level of statistical accuracy provided by the LAT
data, the most important source of uncertainty, often overlooked so far, is due
to the optical depth correction applied to derive the column densities of H I.
Reliable determinations of the amount of atomic gas in the plane are key to
better probe the properties of CRs in the Galaxy.
