We are moving through a paradigm shift in how we view the relationship between Homo Sapiens and microbes-we may be best considered as a mammalian-microbe symbiotic ecosystem.This changing view will impact upon many fields,including psychology and mental health.It is increasingly clear that the Western lifestyle is leading to a dramatic change in our microbiota.This may have causal implications for the dramatic rises we are witnessing in obesity,autoimmune diseases and mental disorders such as autism.It will further impact upon how we care for an ageing population,overcome malnutrition,and deal with infectious diseases in the post-antibiotic era.