Abstract. I draw a distinction between Modeling for Numbers, which aims to address how much, when, and where questions, and Modeling for Understanding, which aims to address how and why questions. For-numbers models are often empirical, which can be more accurate than their mechanistic analogues as long as they are well calibrated and predictions are made within the domain of the calibration data. To extrapolate beyond the domain of available system-level data, for-numbers models should be mechanistic, relying on the ability to calibrate to the system components even if it is not possible to calibrate to the system itself. However, development of a mechanistic model that is reliable depends on an adequate understanding of the system. This understanding is best advanced using a for-understanding modeling approach. To address how and why questions, for-understanding models have to be mechanistic. The best of these forunderstanding models are focused on specific questions, stripped of extraneous detail, and elegantly simple. Once the mechanisms are well understood, one can then decide if the benefits of incorporating the mechanism in a for-numbers model is worth the added complexity and the uncertainty associated with estimating the additional model parameters.Introduction. I draw a distinction between two types of modeling that actually represent extremes on a continuum. The first I call Modeling for Numbers. The questions addressed using these models can be summarized as: How much, where, and when? For example, how much carbon will be sequestered or released, by which parts of the biosphere, on what time course over the next 100 years (e.g., Cramer and others 2001)? The use of these models is clearly important; they address pressing environmental issues and attract a large amount of research money and effort. The second type of modeling I call Modeling for Understanding. The questions addressed with these models can be summarized as: How and why? For example, why can there be only one species per limiting factor (Levin 1970)? These for-understanding questions are more qualitative than the for-numbers questions. The emphasis of modeling for understanding is to understand underlying mechanisms, often by stripping away extraneous detail and thereby sacrificing quantitative accuracy. Modeling for understanding is at least as important as modeling for numbers (Ågren and Bosatta 1990), although the application to pressing ecological issues might be less direct.