Proteins are the ultimate players in the processes that allow an organism to function and reproduce. Their use in medical therapy requires their isolation in pure form. This chapter addresses the calculations needed to quantify protein and, for the cases of several examples, to assess protein activity. How to calculate a protein's molecular weight by its amino acid sequence, by sodium dodecyl sulfate polyacrylamide gel electrophoresis, and by gel filtration are shown. How to quantify protein amount by absorbance using its extinction coefficient and by colorimetric assay, how to use the mathematics involved in labeling proteins with fluorescent dyes, and how to assess a protein's purity by thin-layer chromatography are discussed. Methods for calculating the activity of β-galactosidase, chloramphenicol acetyltransferase, luciferase, and DNA polymerase are shown. The calculations used for monitoring in vitro translation and for determining a protein's isoelectric point are revealed.