Beer bitterness mainly comes from α-acids in hops, which convert to iso-α-acids during boiling. This transformation is affected by factors like pH, time, and gravity. Bitterness is measured in International Bitterness Units (IBU), a standard used especially in industrial brewing for quality control and consistency. While IBU provides a technical measure of bitterness, its relevance in craft brewing is debated, as taste perception also depends on aroma, alcohol content, and individual sensitivity. Despite criticism, IBU remains a valuable tool for recipe design and is essential in both craft and industrial contexts, though for different purposes.