Tammann: The question of the mean mass density in the Universe has been mentioned many times and the organisers thought it would be useful to look at future prospects for obtaining improved estimates of Ω. the methods may be split into two types. First, the conventional methods involve determining the luminosity density in the Universe and multiplying by an appropriate mass-to-luminosity ratio. It must be emphasised that the mass determinations are dynamical and that most of the mass is not visible. There is good agreement among independent workers about the luminosity density and this figure seems to be known within a factor of 2. the appropriate mean value of M/L is more controversial, values between 10 and 200 having been discussed in the preceding lectures. It is agreed, however, that if the mean value of M/L lies in this range, one cannot close the Universe. Perhaps one should be more cautious and say that even granted the uncertainties in the quantities involved, it is unlikely that the Universe is closed.