This paper describes the extent to which the science content in the Next Generation Science Standards (NGSS) is similar to that in the science assessment frameworks for the National Assessment of Educational Progress (NAEP) and the Trends in International Mathematics and Science Study (TIMSS) at the middle school level. Using NAEP as a common reference point, the paper identifies areas of similarity across the three frameworks as well as where NGSS, NAEP and TIMSS may make unique contributions to the assessment of science. The results can inform discussions on the development of NAEP and state assessment systems in science, and in the interpretation of the results of existing (and future) assessments.