The counts derived from the five mid-IR 15 μm (12–18μm LW3 band) ISOCAM Guaranteed Time Extragalactic Surveys performed in the regions of the Lockman Hole and Marano Field, the HDF-North and South (plus flanking fields), together with those of the lensing cluster A2390 at low fluxes and those of IRAS at high fluxes, cover four decades in flux from 50 μJy to ~0.3 Jy. The roughly 1000 sources detected with ISOCAM, 600 of which have a flux above the 80 % completeness limit, guarantee a very high statistical significance for the integral and differential source counts from 0.1 mJy up to ~5 mJy. The slope of the differential counts is very steep (α = −3.0) in the flux range 0.4-4 mJy, hence much above the Euclidean expectation of α = −2.5. When compared with no-evolution models based on IRAS, our counts show a factor ~ 10 excess at 400 μJy, and a fast convergence, with α = −1.6 at lower fluxes.

Multiwavelength studies of a subsample of the ISOCAM sources in HDF and CFRS fields indicate that they are intrinsically bright galaxies (Luminous Infrared Galaxies, LIRGs), with median redshift 0.7. These galaxies, despite their low surface density, are responsible for a large part of star formation at z < 1 and contribute substantially to the cosmic infrared background at 140 μm.