The existence of uncertainties is often mentioned as a crucial limitation for a clear interpretation of Life-Cycle Assessment and Risk Assessment results. Due to this problem, uncertainty analysis is slowly gaining importance in the realization of both assessments, but its use is not common practice. Here, Monte Carlo simulation is used with a procedure that consists of the selection of essential parameters, determination of probability distributions, the simulation itself, significance analysis and interpretation of the results. Examples presented in this chapter include the uncertainties in the application of risk assessment to PCDD/FS and nanomaterials, and of Life-Cycle Assessment and Impact Pathway analysis to a case study on waste incineration in Tarragona/Spain. With the use of the obtained probability distributions for the essential factors in a Monte Carlo Simulation, the results were transformed from a concrete value into a probability distribution around a mean value. The probability distributions obtained correspond to a better understanding of the magnitude of the uncertainties in the results.