The microbiome has been called our "second genome." It is estimated that there are as many microbial cells as human cells in our bodies, and 100 microbial genes for every one human gene. Perhaps even more than the genome, the plasticity of this "second genome" leaves large scope for social and environmental influences across the life course. This talk will provide an overview of current knowledge of the social determinants of the microbiome and invite discussion of how social and population scientists can take lessons from work in sociogenomics to contribute to this emerging field.