I will discuss mathematical models of parts of liver metabolism that relate to public health including folate metabolism, neural tube defects, cancer chemotherapy, DNA methylation, colon cancer and folate supplementation, arsenic detoxification, and the toxicity of acetaminophen. What makes a model good? How do you determine parameters?  How do you conduct biological experiments with models? As a mathematician, how do you figure out what problems to work on and what people to talk to? These hard questions are the subtext of the lecture.