Can dark energy be decaying?

We explore the fate of the universe given the possibility that the density associated with `dark energy' may decay slowly with time. We model decaying dark energy by a homogeneous scalar field which couples minimally to gravity and whose potential has {\ at least one} local maximum. Dark energy decays as the scalar field rolls down its potential and this allows for several interesting possibilities including: (i) the current acceleration epoch can be a transient, (ii) a spatially flat universe ultimately collapses under the influence of a growing negative potential. (The second possibility is realized by the cosine potential.) We examine both possibilities by comparing our model universe with observations of high redshift type Ia supernovae. A maximum likelihood analysis reveals that such models are viable if the effective mass squared near the potential maximum is small (m \~ H₀) and if the initial field displacement is not very large \φ/MP \ 1. The time left until the universe collapses is always larger than \~ 22 Gyrs for \ = 0.3 and H₀ \ 70 km/sec/Mpc (at the 95.4% confidence level).