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ABSTRACT. In this review, the status of measurements of the matter density (m), the vacuum energy density or cosmological constant (), the Hubble constant (H0), and ages of the oldest measured objects (t0) are summarized. Measurements of the statistics of gravitational lenses and strong gravitational lensing are discussed in the context of limits on . Three separate routes to 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. Globular-cluster ages plus a new age measurement based on radioactive dating of thorium in a metal-poor star are briefly summarized. Limits on the product of H0 t0 are also discussed. Many recent, independent dynamical measurements are yielding a low value for the matter density (m ~ 0.2-0.3). A wide range of Hubble constant measurements appear to be converging in the range of 60-80 km/sec/Mpc. 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.
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