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We report on the gamma-ray activity of the high-synchrotron-peaked BLLacertae object Mrk 421 during the first 1.5 years of Fermi operation, from2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT)gamma-ray spectrum above 0.3 GeV can be well-described by a power-law functionwith photon index Gamma=1.78 +/- 0.02 and average photon flux F(>0.3 GeV)=(7.23+/- 0.16) x 10^{-8} ph cm^{-2} s^{-1}. Over this time period, the Fermi-LATspectrum above 0.3 GeV was evaluated on 7-day-long time intervals, showingsignificant variations in the photon flux (up to a factor ~3 from the minimumto the maximum flux), but mild spectral variations. The variability amplitudeat X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially largerthan that in gamma-rays measured by Fermi-LAT, and these two energy ranges arenot significantly correlated. We also present the first results from the4.5-month-long multifrequency campaign on Mrk 421, which included the VLBA,Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations andinstruments which provided excellent temporal and energy coverage of the sourcethroughout the entire campaign (2009 January 19 to 2009 June 1). During thiscampaign, Mrk 421 showed a low activity at all wavebands. The extensivemulti-instrument (radio to TeV) data set provides an unprecedented, completelook at the quiescent spectral energy distribution (SED) for this source. Thebroad band SED was reproduced with a leptonic (one-zone SynchrotronSelf-Compton) and a hadronic model (Synchrotron Proton Blazar). Both frameworksare able to describe the average SED reasonably well, implying comparable jetpowers but very different characteristics for the blazar emission site.

Fermi-LAT Observations of Markarian 421: the Missing Piece of its Spectral Energy Distribution