Measuring cosmological parameters.

In this review, the status of measurements of the matter density (Omegam), the vacuum energy density or cosmological constant (OmegaLambda), the Hubble constant (H0), and the ages of the oldest measured objects (t0) are summarized. Three independent types of methods for measuring the Hubble constant are considered: the measurement of time delays in multiply imaged quasars, the Sunyaev-Zel'dovich effect in clusters, and Cepheid-based extragalactic distances. Many recent independent dynamical measurements are yielding a low value for the matter density (Omegam approximately 0.2-0.3). A wide range of Hubble constant measurements appear to be converging in the range of 60-80 km/sec per megaparsec. Areas where future improvements are likely to be made soon are highlighted-in particular, measurements of anisotropies in the cosmic microwave background. Particular attention is paid to sources of systematic error and the assumptions that underlie many of the measurement methods.