In their seminal paper on environmental enrichment, Mellen and MacPhee (2001) proposed a set of broad goals for enrichment in zoological settings, as well as a framework for enrichment programs. Since that time, the philosophy and practice of environmental enrichment in zoos has continued to grow. Here we review evaluations of enrichment efficacy in the literature since 2001, looking for trends in species, target behaviors, enrichment strategies, and analytic techniques and discussing progress toward the SPIDER vision and outstanding needs in the field. We selected 94 peer-reviewed and 121 non-peer-reviewed articles for review, representing enrichment strategies across a wide range of species. The number of peer-reviewed articles published per year was relatively stable, such that the cumulative number of articles has continued to rise over the thirteen-year review period. We echo the call issued by a number of authors for continued and improved evaluation of enrichment efficacy, and add a recommendation for further exploration of single-subject experimental designs. We also call for focus on a broader array of species and on specific areas of application including reintroduction.