The preparation of this collection arose out of discussions at a meeting of the Council of the International Union of Pure and Applied Physics in 1998. The Commission on Physics Education, C14, had raised the question of whether IUPAP would arrange any special activity to celebrate the millennium. It was decided to prepare a modest publication by asking the chair of each of the commissions to contribute an article of about 2000 words about the field of physics covered by that commission. The editors asked each author to write for physicists who are not specialists in their commission's field. The results form this volume.