Development of unconventional resource plays traditionally were completed using the "plug and perforate" the method (plug-n-perf). In recent years, however, multi-stage fracturing sleeves have seen growing industry acceptance as an alternative completion method to plug-n-perf and is now being employed with increasing frequency with both cement and openhole isolation methods in unconventional resource plays. This type of system is operated by dropping a ball from the surface that seats in a landing baffle to actuate the sleeve and allow for fracturing of the formation. These balls and baffles often can be removed from the ID of the casing string by milling, post frac to remove possible restrictions. However, there are situations that can affect the successful milling of the balls and baffles. This paper explores the conditions that can affect the ball and baffle millout process of multi-stage fracturing sleeves. Different aspects of the milling process will be reviewed to determine the critical elements that must be taken into consideration when milling the balls and baffles. Specific factors include multi-stage fracturing sleeve dimensions, wellbore trajectory, torque and drag, depth location, mill design, weight-on-bit (WOB), viscous pill sweep frequency, and other milling procedures. The investigation of the millout of 185 multi-stage fracturing sleeves in Eagle Ford Shale well completions will analyze these factors, which then will be contrasted with surface millout testing on over 100 multi-stage fracturing sleeves performed on a custom millout testing machine. The surface testing allowed visual observation of millout processes and real-time changing of millout variables that reduced risk and lowered operating cost. Both sets of data will then be analyzed to illustrate the critical factors for successful millout operations and discuss the solutions to the millout challenges.