A hearing instrument is a miniature sound amplifier. But if it only amplified sound, its use would be very limited. Each hearing loss is unique to the individual. Some people have hearing loss in the high pitches. Others have hearing losses in the middle to low pitches. Each person also has variable comfort levels when it comes to the loudness of sound. Thus,  a hearing instrument must be highly selective in how it amplifies sound.

Advanced hearing instruments use programmable technology. Hearing test results are entered into a computer and hearing healthcare professionals then use specialised software, to adjust the hearing instrument to the patient's individual needs. These hearing instruments are often called "analogue programmable" or "digitally programmable" devices.

More advanced hearing instruments are labeled "fully digital" or "simply digital". These instruments can be programmed by a computer and contain a microprocessor. Digital hearing aids translate sounds into digital code. This code is processed according to the users needs and converted back into sound. These sound advanced processing capabilities would be impossible without the microprocessor and the digital signal processing.