The ttbar process is one of the main backgrounds for the H->WW->lnulnu signal search. The simulation of this background as well as an estimation of its contribution to the total systematic error for this Higgs search will be studied in detail. For this, the predictions of the PYTHIA, HERWIG, TopREX and MC@NLO Monte Carlos are compared in order to estimate the effect of different showering programs and of the spin correlations. Furthermore, the question of how to include NLO corrections will be addressed and the simulation of single top background at NLO discussed. In order to extrapolate the ttbar background to the Higgs signal region, once data is available, different normalization methods will be proposed and compared. The experimental uncertainties coming from different normalization processes will be estimated using a full CMS simulation
