* The Cell Theory: The core of biology is the cell theory, which states that all living organisms are composed of cells. This is a fundamental principle, and it holds true for the vast majority of life on Earth.

* Exceptions: The only known exceptions to this are viruses. Viruses are not considered living organisms because they lack the ability to reproduce independently and require a host cell to replicate.

So, while we can say that almost all living organisms are composed of cells, it's important to remember the special case of viruses.