Our ability to understand, predict, and manipulate the behavior of living systems is severely limited by cellular individuality: Genetically identical cells, within a uniform environment, nevertheless exhibit heterogeneous behavior. Studies over the last two decades have highlighted the stochastic origins of cellular individuality, by demonstrating that the inherent randomness of single-molecule events can be amplified into copy-number fluctuations at the cellular level. The picture that emerged is of living cells as “noisy machines”, whose behavior can only be described probabilistically. However, an important caveat to the stochastic view of cellular individuality is that what appears as noise may, in fact, reflect the presence of deterministic drivers of cell-to-cell differences. Failure to detect these “hidden variables” makes cellular behavior appear more random that it truly is. Conversely, properly accounting for them would reveal the precision at which living cells function.