The nucleus is found inside the cells of eukaryotic organisms, which includes plants, animals, fungi, and protists.

Here's a more detailed explanation:

* Eukaryotic cells are complex cells that contain a membrane-bound nucleus. The nucleus is the most prominent organelle within the cell, and it houses the cell's genetic material, the DNA.

* Prokaryotic cells, on the other hand, are simpler cells like bacteria and archaea. They lack a true nucleus and their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

So, to summarize, the nucleus is found:

* Inside eukaryotic cells

* Not present in prokaryotic cells