This is great, I'm of the similar belief that Illustrator is an excellent design tool for UI. The biggest advantage is that it's cross-platform as well.

This is a really good summary of why Illustrator is a good tool for UI design. I already work in a similar fashion, but with a 4x4 grid rather than 5x5. The rasterization trick for imported raster images is something I'll need to play around with. I'd be curious to know how their workflow changes now that Illustrator officially supports asset exporting in multiple resolutions.

( Somewhat related, I wrote a blog post that goes over one extra neat trick involving quickly toggling Select by Path Only, which is my single favorite feature of Illustrator. http://blog.stablekernel.com/illustrator-ux-workflow-tips-for-wireframing-and-design )

Having worked at Area 17 in the past, I loved that they taught these techniques to developers too, front end and backend.

The philosophies are important to the whole company not just the designers.

Wow. A lot of work went into documenting this. Impressive.

It almost makes me want to switch to Illustrator from Sketch. Requires a bit more setup, but once you have that environment ready, it's actually a great tool for UI design.

What is particularly appealing is the use of CC Library, which is the biggest issue with Sketch - managing and sharing assets and UI components with a team and across files.

Yes, there is Craft and other plugins, but I'm wary of relying on third party plugins that don't necessarily have a secure future. Illustrator has all this built in, and Adobe is definitely not going anywhere...

Always loved their work, this is a lovely guide of their philosophy!