In this paper, we propose a novel area of research referred to as singing information processing. To shape the concept of this area, we first introduce singing understanding systems for synchronizing between vocal melody and corresponding lyrics, identifying the singer name, evaluating singing skills, creating hyperlinks between phrases in the lyrics of songs, and detecting breath sounds. We then introduce music information retrieval systems based on similarity of vocal melody timbre and vocal percussion, and singing synthesis systems. Common signal processing techniques for modeling singing voices that are used in these systems, such as techniques for extracting the vocal melody from polyphonic music recordings and modeling the lyrics by using phoneme HMMs for singing voices, are discussed.