In this Letter we derive exact closed-form expressions for the outage probability (OP) in η-μ fading channels. First, a general expression in terms of the confluent Lauricella function is derived for arbitrary values of μ. Next, we restrict the analysis to physical η-μ channel models, i.e. to integer values of 2μ, and obtain exact closed-form expressions for the OP in terms of Marcum Q, Bessel and elementary functions. The results in this Letter are applicable to the OP analysis of maximal ratio combining (MRC) over i.i.d. η-μ or Hoyt fading channels.