Any observant teacher of computer science at secondary or post-secondary level in an English-speaking country has probably noticed that there are considerably more boys than girls in the class. Many are concerned that there are able girls who would enjoy the challenge of a computing career who are choosing not to study computer science, students who may be more capable than many of the boys selecting these studies. The aim of this paper is to look at some of the factors which may influences the students' decisions to study computer science: the nature of the workforce, the prevalence of male and female role models, stereotyping of computing careers, culturally specific factors, perceived gender differences in ability, attitudes to computing, the organization of computing classes, approaches to assessment, and mentoring.