Registration is optional but some journals require it. The most highly-used register is PROSPERO, the International Prospective Register of Systematic Reviews. It aims to provide a comprehensive list of completed and in-progress systematic reviews. It helps avoid unplanned duplication and allows you to see what methods other researchers have used. The questions in the online form help you plan your review and include questions about which types of studies you'll include, primary and secondary outcomes, inclusion/exclusion criteria, and how you will assess risk of bias, synthesize data, etc.