We examine the class of barotropic fluid models of dark energy , in which the pressure is an explicit function of the density , p = f ( \rho ) . Through general physical considerations we constrain the asymptotic past and future behaviors and show that this class is equivalent to the sum of a cosmological constant and a decelerating perfect fluid , or “ aether ” , with w _ { AE } \geq 0 . Barotropic models give substantially disjoint predictions from quintessence , except in the limit of \Lambda CDM . They are also interesting in that they simultaneously can ameliorate the coincidence problem and yet “ predict ” a value of w \approx - 1 .