We review the relationship between tracer distributions and transport timescales in the stratosphere and discuss the use of timescales to evaluate and constrain theories and numerical models of the stratosphere. The age spectrum, the distribution of transit times since stratospheric air last made tropospheric contact, provides a way to understand the transport information of tracers, how sensitive different tracers are to various transport processes, and how to use tracers in combination to constrain transport rates. Trace gas observations can be used to infer aspects of the age spectrum, most commonly the mean age, but also the shape of the spectrum. Observational inferences of transport timescales provide stringent tests of numerical models independent of photochemistry, and comparisons of these observations with chemical transport models have highlighted certain problems with transport in the models. Age simulations and comparisons with data can now be considered standard tests of stratospheric models.