Is it illegal to use spy camera to record someone without their permission? In their home without their knowledge?

Any question on law needs to include your general locale as laws vary by area. More detail would be helpful as well.

If it is their home and you install a camera in their home, then most likely that would be illegal.

Illegal to have your camera inside another persons property. What is the issue?

I can't imagine it being legal anywhere in the world. It's certainly not ethical. If I found out someone did it, you bet I would SUE.

I can't imagine it being legal anywhere in the world. It's certainly not ethical. If I found out someone did it, you bet I would SUE.

No damages, probably. So you can sue but wouldn't get much in the way of a judgment. Better to get the person charged with a crime of some sort.

If you go, into their home, and have the camera on you. In pen, in book, and record video of what happens when together, that is often legal.

You can not plant or put a spy camera in someone else's home. That will often be criminal