Gender Identity: What It Is and Why It Matters is the first book in philosophy to focus on gender identity and transgender rights. To be trans is to have a gender identity different from the gender you were assigned at birth. But what is it to have a gender identity? In the first part of the book, Rach Cosker-Rowland develops a new account of our gender identities as the genders that seem to best fit us. Supported by trans testimony, this subjective fit account explains why gender identities deserve respect, discusses how we can discover our gender identity, and argues for why this is practically important. It also provides an overview of cis and trans, and non-binary and binary, gender identities. In the second part of the book, a new view of trans rights to gender marker change, legal gender recognition, gender-affirming healthcare, and sporting participation and participation is developed. Cosker-Rowland presents an integrity-based account, showing how these trans rights arise from basic liberal rights to live with integrity, to live in line with your judgements of how you ought to live, and what a good or meaningful life for you involves. Rights to live with integrity ground basic liberal rights to freedom of religious belief and expression; this book argues that they also ground trans rights. Finally, Cosker-Rowland addresses a wide range of gender-critical feminist philosophers' views against trans rights and shows that these arguments fail.