Princeton University Observatory, Princeton, NJ 08544--1001, USA E-mail: email@example.com
Invited talk, presented at the 12th IAP Colloquium: Variable Stars and the Astrophysical Returns of Microlensing Surveys.
This is a personal review of various issues related to massive photometric and astrometric searches. A complete inventory of variable stars down to almost any magnitude limit will improve our understanding of the stellar evolution and the galactic structure. A search for detached eclipsing binaries will improve the distance scale, the value of the Hubble constant, and the age of the oldest stars. A search for supernovae will help the determination of cosmological parameters $ \Omega $ and $ \Lambda $. A search for microlensing events will provide insight into the stellar mass function, dark matter, and may lead to a discovery of earth-mass planets.