While millions of dollars are spent on traditional professional development each year in the USA, some school districts are trying other means to increase students' test scores. One strategy is hiring mathematics coaches as on-site professional developers. Whereas mathematics coaching is a newly investigated research area and many issues still need to be addressed as schools and school districts implement coaching programs, coaching gives hope to school districts that have struggled with traditional professional development programs. This paper reviews the existing literature on mathematics coaching, focusing on coaches, their work, their preparation, the conditions needed for an effective coaching program, and the impact of coaching in US schools.