Transactions XXVIA of the International Astronomical Union (IAU) provides a comprehensive and authoritative review of what has been achieved in astronomy during the years 2003 to 2005. The insightful and up-to-date reviews are written by the Presidents and Chairpersons of the IAU's different Divisions, Commissions and Working Groups and each focuses on a different subject area. Topics covered in this wide-ranging volume include: fundamental astronomy; the Sun and heliosphere; planetary systems sciences; stars; variable stars; interstellar matter; the Galactic system; galaxies and the Universe; optical and infrared techniques; radio astronomy; space and high energy astrophysics; and other IAU activities. The reviews are written at a level suitable for colleagues in the same field, but they will also be useful for students and researchers wishing to gain an overview beyond their own research area.