The function of most genes is achieved through the production of a protein that binds other molecules and/or catalyzes biochemical reactions inside the cell. A large fraction of the cellular machinery is dedicated to producing these active proteins in a timely manner and in appropriate amounts. Depending on the gene, protein abundance in cells is set to anywhere from 1 copy to around 1 million copies per cell. Despite this large variance, relatively small changes in the expression of any given gene can cause disease and/or drive evolutionary adaptation. Thus, measuring protein abundance across genes, cells, and different environmental conditions has been the subject of extensive research for the past 50 years. However we still lack a clear picture of how much or how little of a protein is necessary to support cell growth and function. Consequently, we know which ingredients are vital for making functional cells and but not how much is needed. Evolution can change the ingredients and their quantity, but we do not know which mechanism is favored and why. We join our expertise in evolutionary biology (Landry), cell biology (Moriya) and molecular biophysics (Reynolds) to understand how molecular-scale variation in protein activity and abundance contributes to the organism-level fitness (growth) of cells. Our goals are to: (1) determine how changes in protein activity and abundance influence growth rate within a biophysical framework (2) examine how natural systems navigate this space through mutations and selection and (3) better understand why sometimes evolution is limited in what it can change because cells cannot overcome the toxicity of some proteins and their mutations. We will combine quantitative, high-throughput experiments (in both the bacterium E. coli and yeast) with computational modeling. Together, our work will provide a quantitative framework for considering the “design” of protein systems. It will also offer a quantification of the conditions within which cellular systems evolve and a mechanistic understanding as to why protein abundance can range several order of magnitudes while a given protein is allowed to exist in a very tiny abundance window by evolution.