To find out which model best describes our universe, we need to determine the value of the Hubble constant \(H_{0}\) from observations, as well as the fractions of the different energy densities in the total energy density of the universe. However, these quantities cannot be measured directly, but must be derived from other quantities. For example, if you want to determine the Hubble constant by observing distant galaxies, one is facing the problem that, although the brightness and spectrum of such a galaxy can be measured, yet the distance of the galaxy cannot be determined directly. Therefore, in this chapter we will derive relations between actually measurable quantities and the quantities that are relevant in theory. This will allow us to deduce the dynamics of our universe from the results of actual observations.