In real-life situations, quantities and their rate of change depend on more than one variable. For example, the rabbit population, though it may be represented by a single number, depends on the size of predator populations and the availability of food. In order to represent and study such complicated problems we need to use more than one dependent variable and more than one equation. Systems of differential equations are the tools to use. As with the first-order differential equations, the techniques for studying systems fall into the following three categories: analytic, qualitative, and numeric.