This JOSS-accepted paper introduces the R package modeLLtest. modeLLtest implements a variety of tests using leave-one-out cross-validation (LOOCV) to adjudicate among estimation methods. It implements an unbiased comparison of two linear parametric, non-nested models (Harden and Desmarais 2011, State Politics & Policy Quarterly, 11(4): 371-389; Desmarais and Harden 2014, Quality & Quantity 48(4): 2155-2173), a test comparing two estimations of the Cox proportional hazards model - the conventional partial likelihood maximization (PLM) and the iteratively reweighted robust model (IRR) (Desmarais and Harden 2012, Political Analysis 20(1): 113-135), and a set of more general functions for comparison of arbitrary models.