To what extent is it possible to predict the phenotypic differences amongst individuals from their completely sequenced genomes? We use model organisms (particularly C. elegans) and computational analyses to understand when you can, and why you cannot, predict the biology of an individual from their genome sequence alone. Individuals are thankfully all different, and our aim is to understand and predict how these differences originate in the interactions between genetic, environmental, life history, parental and stochastic sources of variation.