Leaves produced at different times of shoot development are often morphologically distinct. Some of these traits change gradually throughout shoot development, others change early in shoot development and are then expressed more-or-less uniformly, while still others are present at one stage of development and absent at a different stage. These latter two patterns allow the shoot to be divided into several discrete phases, the transition between which is termed "vegetative phase change" Although it is clear that miR156, and possibly miR157, regulate many of the changes that occur during shoot development through repressing SQUAMOSA PROMOTOR BINDING PROTEIN-LIKE (SPL) transcription factors, the function of these miRNAs at specific times in development and in specific leaves is poorly understood. How these miRNAs quantitatively regulate SPLs is also unclear. To address these questions, we characterized the morphological and molecular phenotypes of loss-of-function mutations in MIR156 and MIR157 genes, and measured the absolute amount of miR156 and miR157 in successive leaf primordia. We also quantified the effect of varying miR156/miR157 levels on the RNA and protein abundance of their targets. Our results demonstrate that miR156 and miR157 are functionally distinct, and mediate transcript cleavage and translational repression to different degrees at different SPL genes. We also show that variation in the level of miR156/miR157 only has a significant effect on SPL gene expression when these miRNAs are present at relatively low levels. These results provide a foundation for detailed studies of the molecular mechanism of miR156/miR157 activity, and their role in shoot morphogenesis. This knowledge also inspired us to perform a sensitized mutant screen using miR157 mutants as a genetic background. This screen identified the B3 domain transcription factor VALs as regulators of miR156 during vegetative development. val mutations led to increased miR156 abundance as a result of elevated pri-miR156A and pri-miR156C transcript levels, accompanied by reduced H3K27me3 at the MIR156A and MIR156C loci.