Breastfeeding is an unequalled way of providing ideal food for the healthy growth and development of infants; it is also an integral part of the reproductive process with important implications for the health of mothers. Review of evidence has shown that, on a population basis, exclusive breastfeeding for 6 months is the optimal way of feeding infants. Thereafter infants should receive complementary foods with continued breastfeeding up to 2 years of age or beyond.

The WHO says it.  World Health Organization

The AAP says it.  American Academy of Pediatrics

UNICEF says it:  United Nations Childrens Fund

The APHA says it:  American Public Health Association

SO does the American College of Nurse-Midwives, the Academy of Breastfeeding Medicine and the American College of Gynecologists.

The Surgeon General says it.

Nancy Mohrbacher agrees with it.  Nancy Mohrbacher also says that there are some barriers that can make exclusive breastfeeding a difficult goal for mothers.  Read her thoughts:

http://www.nancymohrbacher.com/blog/2011/10/2/no-such-thing-as-a-free-lunch.html