Language processing can be measured objectively using late components in the evoked brain potential. The most established component in this area of research is the N400 component, a negativity that peaks at about 400 ms after stimulus onset with a centro-parietal maximum. It reflects semantic processing. Its presence, as well as its temporal and quantitative expression, allows to conclude about the quality of processing. It is therefore suitable for measuring speech comprehension in special populations, such as cochlear implant (CI) users. The following is an overview of the use of the N400 component as a tool for studying language processes in CI users. We present studies with adult CI users, where the N400 reflects the quality of speech comprehension with the new hearing device and we present studies with children where the emergence of the N400 component reflects the acquisition of their very first vocabulary.