Many numerical techniques developed for analyzing circuits can be “recycled”—that is, they can be used to analyze mass-action kinetics (MAK) models of biological processes. But the recycling must be judicious, as the differences in behavior between typical circuits and typical MAK models can impact a numerical technique’s accuracy and efficiency. In this chapter, we compare circuits and MAK models from this numerical perspective, using illustrative examples, theoretical comparisons of properties such as conservation and invariance of the non-negative orthant, as well as computational results from biological system models.