Ocean Forecasting has started in the eighties in limited ocean regions and now it is a reality in the global open ocean and coastal regions. Predictability of oceanic flows is connected, as for the atmosphere, to initial condition inaccuracies and to model representation errors, both numerical and process wise. To this common set of uncertainty sources the ocean forecasting adds the uncertainty about atmospheric forcing and especially the wind stress, the major forcing of the ocean circulation. The effects of uncertainties is reviewed and forecast errors discussed for the Mediterranean Sea where a real time ocean-prediction system is in place.