The human microbiome consists of the genes contained in the ~100 trillion symbiotic microbial cells, or symbionts, that inhabit our bodies. It has recently been linked to diseases ranging from obesity to autism, and to physiological processes ranging from exercise to sleep. Knight will give an overview of some of the ways we find out about our microbial symbionts, how we establish causal relationships, how we measure change over time, and which features of normal lifestyle and of disease have large and small effects. Interestingly, the effects of technical variation in how these measurements are made can be large, often outweighing the biological effects being sought after. However, many of the biological effects are large enough that even self-collected samples sent in by citizen-scientists are providing useful data in the American Gut Project.