This article describes the process and context of the development of the Best Practice Guidelines (BPG) for working with clients with diverse sex characteristics, sexualities, and genders in Aotearoa/New Zealand and a verbatim section from the BPG is included. Changes in theory, research and applied practice in the discipline of psychology are briefly outlined, along with an account of the process of development and publication of the BPG. The BPG provide guidance on basic awareness, knowledge and skills, and a competence framework for registered psychologists to work with clients with diverse sex characteristics, sexualities and genders. Guidelines published by the New Zealand Psychologists Board are underpinned by the Code of Ethics and aim to support the delivery of evidence based, competent and ethical psychological practice.