The auditory system retrieves, categorizes, and processes relevant information in a complex superposition of acoustic signals. Two processing steps are of special importance and hence subject of the present book: Echo suppression and auditory-object formation. Echo suppression is dealt with by a mathematical analysis of an optimal model derived from a general framework of optimal stimulus reconstruction. We relate the ensuing model to biological and technical strategies and, as a consequence, present new possibilities for both. In the context of auditory object formation, frequency decomposition in the cochlea as well as amplitude modulation are a means of categorizing signal components. The latter is therefore treated by an in-depth analysis of four possible neuronal archetype models. The analysis quantifies limitations and potentialities of neuronal identification of amplitude modulation and periodicity in general.