Prejudice against lesbian, gay, bisexual and transgender (LGBT) people has been researched by social psychologists since the advent of ‘gay affirmative’ psychology in the 1970s. After outlining the groups under the acronym, this chapter examines covert and overt forms of anti-LGBT prejudice. The chapter discusses how all forms of discrimination and prejudice impacting LGBT people can be understood through the theoretical concepts of homophobia, heterosexism, heteronormativity, cisgenderism, minority stress and decompensation. Taken together these concepts account for why anti-LGBT prejudice, stereotyping, and discrimination occurs. Both the causes and effects of prejudice, stereotyping and discrimination directed towards LGBT people are explored. In conclusion, the chapter focuses on the role that social psychology can play in creating positive social change with respect to LGBT people, especially regarding attitude change. Programmes, training and strategies aimed at increasing positive attitudes are important to both promote social inclusion and mitigate against the negative net effects of marginalisation.