The overall purpose of performing measurements in science is to increase knowledge and document about some physical quantity.The measurand has a true value but,except for straightforward counting situations,the true value can never be fully determined.When a quantity is measured,the outcome depends on the measuring system,the measurement procedure,the skill of the operator,the environment and other effects.In practice the uncertainty on the result may arise from many possible sources,including examples such as incomplete definition,sampling,matrix effects and interferences,environmental conditions,uncertainties of weights and volumetric equipment,reference values,approximations and assumptions incorporated in the measurement method and procedure,and random variation.The objective of the paper is how to identify the sources of uncertainty in measurement and their importance on the result. The overall purpose of performing measurements in science is to increase knowledge and document about some physical quantity. The measurand has a true value but, except for straightforward counting situations, the true value can never be determined. Who is quantity is measured, the outcome depends on the measuring system, the measurement procedure, the skill of the operator, the environment and other effects.In practice the uncertainty on the result may arise from many possible sources, including examples such as incomplete definition, sampling, matrix effects and interferences, environmental conditions, uncertainties of weights and volumetric equipment, reference values, approximations and assumptions incorporated in the measurement method and procedure, and random variation. The objective of the paper is how to identify the sources of uncertainty in measurement and their importance on the result.