The sum of 555,555 and 123,456 is:

555,555 + 123,456 = 679,011

Would you like further details or help with another problem? Here are some related questions you may find interesting:

1. What is the result of 678,912 + 111,111?
2. How do you perform long addition with large numbers?
3. What are some shortcuts for adding numbers quickly in your head?
4. How would you subtract 123,456 from 555,555?
5. What is the relationship between addition and multiplication?

Tip: When adding large numbers, it's helpful to break them down into smaller parts to make mental calculations easier (like adding hundreds, then thousands, etc.).