Essentially all your life, you’ve been doing arithmetic with “decimal” numbers, indicating that numbers are represented using ten digits, 0 through 9 (“deci-” is a Greek prefix meaning “ten”). It turns out that numbers can be represented using as many digits as one likes! In what follows, we introduce binary numbers, which are numbers represented using only two digits, namely 0 and 1. We also show how one can leverage the arithmetic techniques you learned when you were 5-8 years old to add, subtract, multiply, and divide binary numbers.