If I lived in Missouri on December 31, 2023, but my permanent residence was in Illinois, do I put that my resident state was Missouri or Illinois?

[Vanessa A]

Is Missouri a second home or a temporary absence?  Did you maintain your home in Illinois?

If you maintained your home in IL and MO is either a second home or was just a temporary residence, then you would your resident state as IL.

If you lived in MO and earned money in IL or vice versa, you would need to file a Part year resident return for MO (or non resident if you never set up a home there) as these states do not have reciprocal agreements.