Hydrogenated amorphous carbon (a-C:H) is a class of amorphous carbon with more than 30% hydrogen content and containing sp2 as well as sp3 carbon atoms. It is widely used as a hard mask in semiconductor device fabrication, protective coatings, lubricants, and biomedical applications. The properties of a-C:H films are known to be strongly dependent on the carbon bonding structure and are characterized using the sp2/sp3 carbon hybridization ratio. The a-C:H films are typically deposited by plasma-enhanced chemical vapor deposition (PECVD) processes, and this review summarizes and discusses the relationship between the sp2/sp3 ratio of a-C:H and plasma characteristics. The effects of temperature, radical density, ion density, and ion energy on the sp2/sp3 ratio of a-C:H are investigated and summarized.