A mastering engineer is one skilled in the practice of taking audio (typically musical content) that has been previously mixed in either the analog or digital domain as mono, stereo, or multichannel formats and preparing it for use in distribution, whether by physical media such as a CD, vinyl record, or as some method of streaming audio.

The mastering engineer is responsible for a final edit of a product and preparation for manufacturing copies. An audio mastering engineer (especially in music) should have complete knowledge of audio engineering, and may hold an audio or acoustic engineering degree. Most audio engineers master music or speech audio material. The best mastering engineers might possess arrangement and record production skills, allowing them to 'trouble-shoot' mix issues and improve the final sound. Generally, good mastering skills are based on experience which can be the result of many years of practice.

Many mastering engineers use special audio signal chain equipment to process audio. The equipment used in mastering is almost entirely dedicated to this goal and is usually very expensive. Some advocates for digital software claim that plug-ins are capable of handling mastering processes without signal degradation. The quality of the results varies according to the algorithm these programs use to operate. Reversed engineering of the best programs can be applied to create audio software with defects in the math and thus a substandard audio digital rendition. For this reason many engineers feel that hardware and not software should be the last process before CD authoring.