The set of single-gene deletions in yeast can be used to evaluate the effect of mutation on fitness over the whole genome. The measurement of growth in pure culture or relative growth in mixtures has confirmed that most deletions have little effect in laboratory culture. Moreover, there is a sharp distinction between lethality and a very mild impairment of growth, with very few intermediate cases. Different components of fitness, such as growth rate and yield, are positively correlated. Growth is also positively correlated across environments, although new conditions of growth usually identify a few conditionally impaired strains. Double mutants on average show alleviating epistasis, although a few per cent of combinations are synthetic lethal. The properties of the yeast deletion set provide us with the first genome-wide account of fitness, although transferring these conclusions to the field is a task for the future.