Rice blast disease is both the most explosive and potentially damaging disease of the world's rice (Oryza sativa) crop and a model system for research on the molecular mechanisms that fungi use to cause plant disease. The blast fungus, Magnaporthe oryzae, is highly evolved to sense when it is on a leaf surface; to develop a pressurized cell, the appressorium, to punch through the leaf cuticle; and then to hijack living rice cells to assist it in causing disease. Host specificity, determining which plants particular fungal strains can infect, is also an important topic for research. The blast fungus is a moving target, quickly overcoming rice resistance genes we deploy to control it, and recently emerging to cause devastating disease on an entirely new cereal crop, wheat. M. oryzae is highly adaptable, with multiple examples of genetic instability at certain gene loci and in certain genomic regions. Understanding the biology of the fungus in the field, and its potential for genetic and genome variability, is key to keep it from adapting to life in the research laboratory and losing relevance to the significant impact it has on global food security.