Structure and evolution of Giant Molecular Clouds.

This is a review talk on the structure and evolution of Giant Molecular Clouds, delivered at the 1985 General Assemble of the International Astronomical Union. GMCs are formed in the spiral arms of the Galaxy, by agglomeration of small molecular clouds. GMCs grow efficiently, and contain about 1/4 of the mass in the interstellar medium. After a lifetime of about 100 million years, GMCs are destroyed by the bright young stars that form within them.