  There are now evidences that the cosmological constant $\Lambda$ has a
non-zero positive value. Alternative scenarios to a pure cosmological constant
model are provided by quintessence, an effective negative pressure fluid
permeating the universe. Recent results indicate that the energy density $\rho$
and the pressure $p$ of this fluid are constrained by $-\rho\leq p
\buildrel<\over\sim-0.6 \rho$. Since $p=-\rho$ is equivalent to the pure
cosmological constant model, it is appropriate to analyze this particular, but
important, case further. We study the linear theory of perturbations in a
Friedmann-Robertson-Walker universe with a cosmological constant. We obtain the
equations for the evolution of the perturbations in the fully relativistic
case, for which we analyze the single-fluid and the two-fluid cases. We obtain
solutions to these equations in appropriate limits. We also study the Newtonian
approximation. We find that for a positive cosmological constant universe (i)
the perturbations will grow slower in the relativistic regime for a two-fluid
composed of dark matter and radiation, and (ii) in the Newtonian regime, the
perturbations stop growing.
