Class 1 KNOTTED1-LIKE HOMEOBOX (KNOXI) genes encode transcription factors that are expressed in the shoot apical meristem (SAM) and are essential for SAM maintenance. In some species with compound leaves, including tomato (Solanum lycopersicum), KNOXI genes are also expressed during leaf development and affect leaf morphology. To dissect the role of KNOXI proteins in leaf patterning, we expressed in tomato leaves a fusion of the tomato KNOXI gene Tkn2 with a sequence encoding a repressor domain, expected to repress common targets of tomato KNOXI proteins. This resulted in the formation of small, narrow, and simple leaves due to accelerated differentiation. Overexpression of the wild-type form of Tkn1 or Tkn2 in young leaves also resulted in narrow and simple leaves, but in this case, leaf development was blocked at the initiation stage. Expression of Tkn1 or Tkn2 during a series of spatial and temporal windows in leaf development identified leaf initiation and primary morphogenesis as specific developmental contexts at which the tomato leaf is responsive to KNOXI activity. arabidopsis thaliana leaves responded to overexpression of arabidopsis or tomato KNOXI genes during the morphogenetic stage but were largely insensitive to their overexpression during leaf initiation. These results imply that KNOXI proteins act at specific stages within the compound-leaf development program to delay maturation and enable leaflet formation, rather than set the compound leaf route.