This article reviews findings on the maternal stress and health effects of lactation. Several significant associations have emerged. Compared with not breastfeeding, breastfeeding is associated with increased parasympathetic nervous system modulation, greater vascular stress response, lower perceived stress levels, and fewer depressive symptoms. Breastfeeding exclusively is associated with an attenuated initial sympathetic cardiac nervous system response to some laboratory stressors. Bottle-feeding is associated with increased sympathetic and decreased parasympathetic cardiac control. The act of breastfeeding is associated with decreased neuroendocrine response to stressors and decreased negative mood. Finally, breastfeeding is associated with enhanced physical and mental health compared with non-breastfeeding.