Over the past five decades, the criteria for defining an MLB ace have shifted significantly, moving from traditional stats like wins and complete games in the 1970s to a focus on strikeouts, control, and efficiency in recent years.

By 1985, the emphasis shifted slightly as aces relied more on secondary pitches and had higher ERA+ due to increased offensive power, leading to fewer innings and complete games.

By 2015, strikeouts became a defining feature of aces, with pitchers like Kershaw and Scherzer demonstrating high-velocity pitches and multiple off-speed options, elevating strikeout rate importance.

As of 2025, aces are characterized by shorter outings with high effectiveness, often allowing very few runs in fewer innings, with WAR being a key measure of their value.

In 1975, aces were characterized by durability and high innings pitched, with pitchers like Gaylord Perry exemplifying this with over 200 innings and numerous complete games.

Starting in 1995, the focus moved towards strikeout ability, with pitchers like Maddux and Johnson excelling despite fewer wins and innings, emphasizing dominance and strikeout rate.

The trend continued into 2005, where strikeout-to-walk ratios rose, with pitchers like Johan Santana and Roy Halladay showcasing control and strikeout prowess, although wins remained a key metric.

In recent years, modern aces like Skenes have high WAR and ERA metrics despite fewer traditional stats, indicating their value is comparable to past legends even if their seasons are less iconic in raw stats.

Overall, the evolution reflects a shift from traditional workload and durability metrics to a modern emphasis on strikeouts, control, and efficiency, driven by changes in game strategy and pitcher management.