Variable importance (VI) tools are typically used to examine the inner workings of prediction models. However, many existing VI measures are not comparable across model types, can obscure implicit assumptions about the data generating distribution, or can give seemingly incoherent results when multiple prediction models fit the data well... In this paper we propose a framework of VI measures for describing how much any model class (e.g. all linear models of dimension p), any model-fitting algorithm (e.g. Ridge regression with fixed regularization parameter), or any individual prediction model (e.g. a single linear model with fixed coefficient vector), relies on covariate(s) of interest. The building block of our approach, Model Reliance (MR), compares a prediction model's expected loss with that model's expected loss on a pair of observations in which the value of the covariate of interest has been switched. Expanding on MR, we propose Model Class Reliance (MCR) as the upper and lower bounds on the degree to which any well-performing prediction model within a class may rely on a variable of interest, or set of variables of interest. Thus, MCR describes reliance on a variable while accounting for the fact that many prediction models, possibly of different parametric forms, may fit the data well. We give probabilistic bounds for MR and MCR, leveraging existing results for U-statistics. These bounds can be generalized to create finite-sample confidence regions for the best-performing models from any class. We also illustrate connections between MR, conditional causal effects, and linear regression coefficients. We outline a binary search procedure to compute estimates of MCR. We then apply MR & MCR in a public dataset of Broward County criminal records to study the reliance of recidivism prediction models on sex and race, with code available at https://github.com/aaronjfisher/mcr. (read more)