The registration portal for the 21st edition of the Mobile and Ubiquitous Multimedia Conference is now open. The situation in Belgium is stationary and coronavirus measures have been relaxed. The current plan is to hold the conference in a hybrid manner, with talks happening both in-person and online. We encourage you to attend the conference physically for the best experience. The registration website is available from https://www.mum-conf.org/2021/registration/ The Early Bird deadline is November 7 2021. You can still contribute to the conference via poster and demos (DL October 8), check the website for more details: https://www.mum-conf.org/2021/cfp-posters/ https://www.mum-conf.org/2021/cfp-demos/ See you in Leuven in a few months!