["Population and Development Review, Volume 46, Issue 4, Page 677-708, December 2020. ", "\nAbstract\nWhile women's labor force participation rates (LFPRs) in the United States stalled over the last quarter‐century, European countries exhibited a variety of trajectories. We draw on demographic and gender theories of women's life course to understand changes in women's LFPR during their prime child‐rearing years. We build expectations about how aggregate trends may be driven by shifts in the prevalence of key demographic events such as child‐rearing (i.e., compositional) versus shifts in the association of these events with women's LFP (i.e., behavioral). We use data from the European Union Labour Force Surveys and the US Current Population Survey in Kitagawa–Blinder–Oaxaca decomposition models to decompose trends in women's LFPR from 1996 to 2016 across 18 countries by educational attainment, partnership status, and parental status for women aged 20–44. Compositional and behavioral shifts positively contribute to higher LFPR in most countries, but lower rates in several others. Behavioral change is not widely shared across groups of women. Partnered mothers without college degrees are the main contributors to behavioral change and show the greatest variability across countries. We suggest greater research attention to this “missing middle,” as their LFP is key to understanding change during this period.\n"]