Genotypes × environment interactions (GEI) are more fully analyzed when genotypes and environments are well characterized. The characterization of the environments via direct measurements seems somewhat immediate. However, this method generates too many variates that reduce their own significance in the GEI analysis. Several methods can be used to reduce this number, e.g., a crop diagnosis combined with probe genotypes and biological indicators. Genotypes can be characterized by several methods: (1) via direct measurements, (2) using crop modelling, or (3) by comparison to probe genotypes. All these methods will be illustrated in this chapter, mainly for winter wheat (Triticum aestivum L.), and their prospects for QTL detection will be discussed.