While Robert Hooke first observed and named cells in 1665, it was Schwann and Schleiden who formulated the major tenets of the theory in the 1830s.

Here's a breakdown:

* Matthias Schleiden (1838) observed that all plants are made of cells.

* Theodor Schwann (1839) extended this idea to animals, proposing that all living things are composed of cells.

* Rudolf Virchow (1855) later added the concept of "omnis cellula e cellula," meaning all cells arise from pre-existing cells.

So, while Hooke's initial observations paved the way, it was the combined efforts of Schwann and Schleiden that truly established the cell theory as a cornerstone of biology.