The 9th IET Renewable Power Generation Conference took place online in March 2021, having originally been intended to take place in Dublin, Ireland some months earlier. A major motivation for the conference was the recognition of the challenges and possibilities arising from increasing renewable shares in many countries, and the critical need for cost-effective, reliable, and robust solutions as part of the ongoing transformation of our power and energy systems. Despite the restrictions of the pandemic, approximately 70 papers were successfully presented at the conference across 15 oral and poster sessions, covering a wide range of themes including low inertia power systems, wind power plant modelling and control, renewable energy forecasting, distribution network operation and long-term planning, power system protection, electricity markets and system services etc. Subsequent to the conference, a number of the authors of the presented papers were invited to submit an extended version of their papers to a special issue of the IET RPG journal. Ultimately, after a thorough peer review process, 16 papers were accepted for the special issue, which have been grouped here under five main themes: electrical network operation and planning; power electronic converter control systems; flexibility/frequency support measures and electricity markets; power system security, protection and monitoring; and tidal stream energy.