The HST Key Project on the Extragalactic Distance Scale: A Search for Three Number -- Calibration of the Distance Scale From Cepheids -- Currents Uncertainties in the Use of Cepheids as Distance Indicators -- The Cepheid Calibration of Type Ia Supernovae as Standard Candles -- RR Lyrae Distance Scale: Theory and Observations -- Globular Cluster Distances From RR Lyrae Stars -- Blue Supergiants as a Tool For Extragalactic Distance - Theoretical Concepts -- Blue Supergiants as a Tool for Extragalactic Distances - Empirical Diagnostics -- Supernovae as Cosmological Standard Candles -- Type Ia -Supernovae and Cosmology Models for Type Ia Supernovae and Cosmology -- Novae as Distance Indicators -- Extragalactic Distances from Planetary Nebulae -- Distances to Local Group Galaxies -- The Globular Cluster Luminosity Function: New Progress in Understanding an Old Distance Indicator.

This volume reviews the current status with respect to both theory and observation of the extragalactic distance scale. A sufficient accuracy is required both for a precise determination of the cosmological parameters and also in order to achieve a better understanding of physical processes in extragalactic systems. The "standard candles", used to set up the extragalactic distance scale, reviewed in this book include cepheid variables, RR Lyrae variables, novae, Type Ia and Type II supernovae as well as globular clusters and planetary nebulae.