The article discusses the role played by decision mechanisms in the leading model of timing, scalar expectancy theory. Examples of the roles played by decision mechanisms in explanations of behaviour on temporal generalization and bisection are presented. Decision mechanisms for different timing tasks often have a common form (thresholded normalized difference, TND), where differences between durations are 'normalized' (i.e., divided) by another duration value, then compared with a threshold. The TND principle provides a rule for both similarity and identity judgements of duration. The role of threshold mechanisms in timing is discussed, and it is shown that some procedural manipulations appear to specifically alter threshold values. Finally, problems in modelling the decision processes involved in verbal estimation are discussed.