First reported by Farnum as much as 40 years ago, diketopyrrolopyrroles (2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-diones) are currently one of the most widely used dyes, with applications that span high-quality pigments, field-effect transistors, bulk-heterojunction solar cells, dye-sensitized solar cells, and fluorescence imaging. Due to the breadth of their utility, there have been several focused reviews on this topic since 1988. Considering the growing importance of these dyes, this review describes the overall progress made in four decades in both synthesis and the elucidation of their reactivity, and offers a critical comparison of various methods reported in the literature. The relationship between their structure and optical properties is also reviewed in detail. The extension of diketopyrrolopyrroles' chromophore leads to a significant change in both linear and nonlinear optical properties. A very strong bathochromic shift in the absorption and an increase in the two-photon absorption cross-section are typical for these analogues. Straightforward synthesis combined with good stability and a near-unity fluorescence quantum yield ensures that diketopyrrolopyrroles will continue to attract attention for years to come. The goal of this review is to systematize knowledge on these dyes, enabling their use in more real-world applications.