LISA and its precursor LISA Pathfinder

The Laser Interferometer Space Antenna (LISA) is a joint ESA-NASA mission designed to observe gravitational waves in the frequency range between 10-4 to 1 Hz, where ground-based detectors are limited by terrestrial noise. Sources in this frequency range include supermassive black holes and galactic binary stars. LISA consists of three identical spacecraft separated by 5 million kilometers carrying a total of six free flying proof masses in heliocentric drag-free orbit. The fluctuations in separation between two test masses located in different satellites will be measured by laser interferometry with picometer precision. LISA Pathfinder is a technology demonstration mission for LISA consisting of only two test masses in one single satellite. It will be launched in 2009, five years before LISA.We provide here an overview of the development of LISA and LISA Pathfinder with particular emphasis on the interferometry