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We present narrow-band imaging of the edge-on spiral NGC 891 in the Hα and [S II] λλ6717, 6731 lines. Emission from H II regions confined to the plane of the galaxy and from diffuse gas up to about 4 kpc off the plane is readily detected. The full radial extent of the diffuse emission in the plane is about 30 kpc. The [S II]/Hα ratio is quite uniform in the halo and is significantly higher in the halo than in the disk-consistent with observations of the diffuse ionized gas in our Galaxy. NGC 891 is found to have a surface density of diffuse ionized gas twice the Galactic value, a thicker ionized gas layer, and a larger surface density of ionized gas relative to neutral gas. We interpret these to be consequences of a relatively high level of star formation in this galaxy. Other star formation tracers indicate the same conclusion. We see many vertical Hα filaments, or "worms," extending to > 2 kpc off the plane of the galaxy. These worms are interpreted in terms of chimney models for the interstellar media of spirals. We also see several "supershells" which are centered up to 1400 pc above the plane. The highest z emission could be due to gas accelerated into the halo by chimneys, but reionization of this gas by another galactic source is probably required. The intergalactic EUV ionizing flux is insufficient to maintain the high-z ionization.