One of the most daunting problems for biochemists is the expression of recombinant proteins. Often, the host organism differs from the organism from which the gene coding for the protein of interest was derived. This article provides guidelines to determine whether or not protein expression is a problem, describes possible reasons for low protein expression, and covers several possible solutions. A protocol for measuring protein expression during E. coli cell growth and after induction is given. The reader should note that low protein expression is a complex problem that often stems from a variety of factors. Combinations of the solutions presented in this article may be required to solve a problem of protein expression. A brief overview of host cell expression systems is given, but the article primarily focuses on expression in E. coli as this is the most commonly used host organism. Some of the methods discussed here, however, may be applied to other expression systems.