Floating-point numbers cannot represent real numbers with infinite precision. Additionally, there are sometimes uncertainties in our knowledge of the true value of a quantity. Both of these situations can be addressed with interval arithmetic that keeps bounds on the possible value of a number while a calculation is in progress. In this chapter, we describe interval arithmetic; implement basic operations for interval arithmetic in C; discuss functions as they pertain to intervals; examine interval implementations for C and Python; and, finally, offer advice on when to use interval arithmetic.