Two separate but related issues are addressed: how simultaneous interpretation (SI) works on a cognitive level and how such translation can be objectively assessed. Both of these issues are discussed in the light of a qualitative and quantitative analysis of a large corpus of SI and shadowing. The knowledge gained from the analysis is used to characterize the cognitive processes concomitant with SI. Discourse-based knowledge is also used to develop three computer systems for the assessment of SI: (i) content-based semi-automated; (ii) time structure-based; and (iii) coherence-based. For each system, several parameters of performance are identified, and they are correlated with assessments rendered by the traditional, subjective, qualitative method. Acoustic discourse analysis, which uses standard signal processing techniques, leads to the conclusion that SI quality can be assessed quantitatively with varying degrees of automation. Proficiency is shown to be directly related to coherence and speech rate but inversely related to omission and delay. High proficiency is associated with a high degree of simultaneity and prudence, but a low degree of time dissipation and inactivity.