This chapter aims at introducing a new type of design of decision support systems (DSSs). The DSS presented here is a software based on client-server technology that enables great accessibility by the Web. Its conception flow has been established to be generic and not explicitly problem-oriented. In this way, once the first DSS is built, the creation of other DSSs will be easy and time-saving. The creation of the DSS requires the collaboration of different experts such as agronomists, computer specialists, and interface experts. Their communication is improved by the use of the formal language Unified Modeling Language (UML) throughout the process of software design. The relevance of the DSS comes from its use of scientific mechanistic models adapted to the users' needs and from a flexible architecture that allows easy software maintenance. The chapter is structured as follows: after the introduction, the second section will explain in detail the methods used to build the scientific models that describe the biological system. The third section describes the methods for the validation and implementation of those models, and the fourth section deals with the transcription of the models into software components processable in the DSS. Finally, the last section of this chapter describes the architecture of the client-server application.