The problem given is to add the following: $-99 + 0$.

The sum is:

$-99 + 0 = -99$

So, the answer is $-99$.

Would you like more details or have any questions?

Here are 5 related questions you might find interesting:

1. What happens when you add zero to any number?
2. How does adding a negative number differ from adding a positive number?
3. What is the additive identity property in mathematics?
4. Can you subtract zero from a number, and what would the result be?
5. How does the concept of zero impact operations in algebra?

Tip: Remember, adding zero to any number does not change the value of that number.