The boundary Harnack principle allows one to compare the behavior of two harmonic solutions when they both vanish on the boundary. This principle has proven useful in PDE, harmonic analysis, and free boundary problems. In this talk I will review the principle and some of its applications. I will then introduce a new boundary Harnack principle for functions with nonzero right hand sides. This new principle has applications to Hele-Shaw flow as well as free boundary problems. This is joint work with Henrik Shahgholian.