Despite the success of the Concordance LCDM model, the nature of the dark energy in the universe still remains unknown. There is hope that the increasing accuracy of the cosmological observations may give a new insight into the physics of this exotic component and eventually confirm or disproof the Cosmological Constant paradigm. This phenomenological approach requires modeling the "unknown". I will show that simple approaches lead to misleading results. Using the most recent compilation of cosmological data we constrain a set of parameters describing the properties of a general dark energy fluid. We find that current observations are not accurate enough to distinguish between a Cosmological Constant and a time dependent form of dark energy. Future measurements of the cross-correlation between CMB and Large Scale Structure data will play a crucial role in the quest for dark energy.