Sex Matters addresses a cluster of related questions that arise from the conflict of interests between rights based on sex and rights based on gender identity. Some of these questions are theoretical, including: who has the more ambitious vision for women's liberation, gender-critical feminists or proponents of gender identity? How does each understand what gender is? What are the arguments for the refrain that 'trans women are women!', and do they succeed? Other questions taken up in the book are more applied to specific issues in law and policy including: Should there be a right to exclude people who are biologically male from women-only spaces? How do the interests of all stakeholders to bathrooms, in particular, trade off when it comes to moving from sex to gender identity as the basis for self-inclusion? If we think about types of transition, or gatekeeping requirements on transition, as providing assurance to women who are asked to accept the opening up of women-only spaces to transwomen, are any such assurances sufficient? Is 'TERF' a slur, as some radical and gender-critical feminists have claimed? And finally, is gender-critical speech 'hate speech', as it has been classified by some social media platforms, or at least harmful speech?