The Infrared Astronomical Satellite (IRAS) has completed an unbiased all-sky survey at wavelengths from 10 to 100 um. The design and performance of the focal plane array is described with emphasis on in-orbit measurements of the sensitivity and stability. In the four broad spectral bands centered at 12, 25, 60 and 100 um the system NEFD values are, in Jy/Hz1/2,0.03, 0.025, 0.046 and 0.21 respectively (Jansky = 1E-26 w/m2/Hz). For point sources, a single scan at the survey rate of 3.8 arcmin/sec yields limiting flux densities at the 3 sigma confidence level of 0.36 , 0.30, 0.39 and 1.2 Jy. The DC stability of the JFET amplifiers and the excellent off-axis rejection of the telescope permit total flux meaurements of extended infrared emission at levels below 1 E 6 Jy/sr. Response of the extrinsic silicon and germanium photodetectors to ionizing radiation is described.