  We use linear and quasi-linear perturbation theory to analyse cold dark
matter models of structure formation in spatially flat models with a
cosmological constant. Both a tilted spectrum of density perturbations and a
significant gravitational wave contribution to the microwave anisotropy are
allowed as possibilities. We provide normalizations of the models to microwave
anisotropies, as given by the four-year {\it COBE} observations, and show how
all the normalization information for such models, including tilt, can be
condensed into a single fitting function which is independent of the value of
the Hubble parameter. We then discuss a wide variety of other types of
observations. We find that a very wide parameter space is available for these
models, provided $\Omega_0$ is greater than about 0.3, and that large-scale
structure observations show no preference for any particular value of
$\Omega_0$ in the range 0.3 to 1.
