Learning curves have been used for over 50 years since T. P. Wright observed the cumulative unit cost to produce an airplane decreased at a predictable rate as the quantity produced increased. Industrial engineers have used learning curves to estimate the higher cost of initial production at different points in time and for different production volumes until the factory workers are able to achieve the industrial engineered standard time established. This chapter defines and explains the underlying factors affecting not being able to achieve standard time and the learning curves used to predict variance from standard, or realization factor, as it is sometimes referred to. Operator learning is only one of the key elements that affect achieving standard time. There is product and process learning and loss of learning due to breaks in production in addition to logistics and technical elements. There is considerable variability in estimating the realization factor applied to standard times. This detailed analysis of the many factors or elements that define why standard time is not met were the foundation of the aerospace and defense industry position in the U.S. Senate hearings on work measurement in 1985. Furthermore, current and future issues impacting learning are discussed, such as the effect of automation, self-empowered workforce, and future trends in product and process innovation. Discussions of both the unit cost and cumulative average cost approaches to learning curves, as well as the basic algebraic logarithm equation, are also...