While CET theory is a starting point, the deficit in unit-level turnover theory remains and impedes understanding of how and why turnover affects unit performance. To address this deficiency, we focus on human capital flows (employee movement in and out of units) to explain how and why movement impacts unit performance. We make four primary contributions. First, we test several central tenets of CET theory, including the importance of the quantity dimension of collective turnover in affecting performance. As Hausknecht and Holwerda recently emphasize, “. . .the quantity of departures—the currency of classic approaches—is clearly fundamental to any argument regarding turnover’s effects” (2013: 211), although turnover researchers generally focus on the quality of human capital losses (e.g., Trevor, Gerhart, & Boudreau, 1997), thus downplaying the quantity dimension.