Phenomics is a relatively new approach by breeders, who use modern sensors for monitoring germplasm performance and as a selection tool. Plant physiologists can also make use of phenotyping in their studies, which usually focus on plant response to abiotic and biotic stresses. Simple active radiometer sensors such as RapidScan enable phenotyping within the framework of field experiments, and may open new horizons for breeders. The objectives of this study were to: (i) test the ability of vegetation indices (VIs) to distinguish between wheat cultivars during the entire growth period; and (ii) evaluate the accuracy of yield estimation based on only one or a few monitoring days. The following studies were conducted to test RapidScan as a tool for wheat phenotyping in the field: (i) response of 10 cultivars to water deficiency; (ii) response of 13 cultivars to late top N application. Plants were scanned weekly. The default VI parameters, NDVI and NDRE, could be used to show development during the growing season. However, due to low repeatability, it was almost impossible to use these indices for cultivar differentiation, unless there was wide variation in the cultivar phenotype. Nevertheless, combining the data from a few monitoring days improved cultivar classification and yield estimation. Although variation between fields and treatments affected the VI vs. crop parameter relationships more than the within-field variation, the results showed that the use of the proximal-sensing technique allows for rapid and quite accurate phenotyping. Thus, RapidScan can assist breeders during breeding programs for wide-scale in-field phenotyping.