Temporal sequences of sounds are transformed into neural activity patterns that propagate through auditory pathways where more centrally-located processors concurrently analyze their form and interpret their meaning and relevance. Thus, a great deal of attention is paid here to analyzing the signal in the time domain. This chapter mainly treats technical aspects of the running autocorrelation function (ACF) of the signal, which contains the envelope and its finer structures as well as the power at the origin of time of the ACF. The ACF has the same information as the power density spectrum of the signal under analysis. From the ACF, however, significant factors may be extracted that are directly related to temporal sensations. The ACF signal representation exists in the auditory pathway, as is discussed in Chapters 4 and 5.