The sensitivity of staring and scanning infrared seekers responding in the 3 - 5 micrometer band is assessed. The signal-to-noise ratio (SNR) for an aircraft target at near head-on aspect is calculated. Results are presented at the moment of lock on, as well as during the free flight of the missile. Noise increase associated with dome heating is well taken into account. Staring seekers are superior in sensitivity compared to scanning seekers, allowing the missile to be launched from longer distance. But in some cases the noise signal grows at faster rate than the target signal along part of the missile route. This is further aggravated by the residual fixed pattern noise (RFPN), which grows in proportion to the dome heating, and eventually becomes the dominating noise. With sapphire-like dome, the focal plane array (FPA) of the staring seeker eventually saturates due to dome heating, unless a narrow spectral band is used, or the integration time is shortened. Reducing the integration time does not substantially affect the SNR as long as the RFPN is the dominating noise. Dome heating may cause rapid increase in the detector dc current, so the usual non-uniformity correction (NUC) procedures may no longer be adequate. One may have to consider correction in real time along with other methods that rely on target motion. These issues are, however, out of the scope of this paper.