When a magnetic liquid (a ferrofluid) is trapped between two closely-spaced glass plates (a Hele-Shaw cell) and subjected to a magnetic field, a remarkable fingering instability takes place. Depending on the parameters of the applied field, the evolution can result in a branched tree-like structure or a more complex labyrinthine pattern. In this talk, I will give an introduction to this type of pattern formation process and present a technique that provides some control over the fingering instability. Much of the work presented in this talk has been accomplished at an undergraduate institution; students are strongly encouraged to attend.