From 23:00 GMT on that date, the UK stopped following EU rules, with new arrangements for travel, trade, immigration and security co-operation coming into force.
A trade deal with the EU was agreed on 24 December, and UK lawmakers voted in favour of the agreement a week later.
The UK's departure means big changes for business - with the UK and EU forming two separate markets - including the end of free movement and new import and export regulations.
We've taken the key points from the government site to give you an idea of what you need to be doing to keep your business running smoothly.
Talk to your ScaleUp 360 account manager as they can help you find the right information and support.
You can also sign up to the government website to get relevant news and alerts.
Key areas to consider are detailed below with a link to the the key points as a fact sheet.
Import - what you need to know if you are bringing in goods to the UK
- Export - what you need to know if you are sending goods out of the UK
- Intellectual Property - including patents and design work
- Personal Data - keeping personal data secure and within processing regulations