<sub>1. *Fluffypony gives a great explanation of why mandatory network upgrades are good for Monero.* ([Monero Missives for the Week of 2016-06-20]({{ site.baseurl_root }}/2016/06/20/monero-missive-for-the-week-of-2016-06-20.html))</sub>
The Monero Core Team plans for a network upgrade roughly every 6 months. At that time, if you are running a node it must be updated to the most recent version of the Monero software or it will no longer be able to participate in the network.
The rules that nodes follow are built into the Monero software; When all nodes agree about the rules to follow this is called @consensus. Consensus is necessary for a cryptocurrency because it is how the blockchain is built; If nodes don't agree about which blocks are valid, for example people who have not updated their Monero software, those nodes that don't agree will no longer be able to participate in the Monero network.
