Models of the root system architecture are useful tools for studying the plant-soil system. They include several interesting characteristics: (i) they simulate both the structure and spatial distribution of the root system; (ii) they allow a straightforward integration of developmental processes at the root level (e.g. elongation, branching) and their interaction with soil properties; (iii) they enable the simulation of root-shoot interactions via plant resources or signals. However, few of them have been integrated into larger crop or ecosystem models, probably because they are too complex, or too specific to given species or stages. A simple and generic model (ArchiSimple) is presented which avoids these drawbacks. This model has been designed to enable the representation of the architectural diversity coming from various plant species interacting with various environmental conditions. In comparison to previous models, it is a new conceptual framework which assumes that most variations of developmental characteristics of the roots can be summarized by the size of their tip. Thus, two robust and important relationships are included: elongation rate versus root tip diameter; diameter of lateral roots versus diameter of their mother roots. These relationships allow representing the distribution and dynamics of root diameters. The model parameters are concrete, and can be estimated independently from data coming either from specific experiments or from the literature. The calibration procedure is illustrated for a number of species and for the main processes, namely emission, elongation and branching. Beyond the acquisition of specific parameters, and their possible comparison, the approach allowed the critical evaluation of the basic sub-models which are included in the whole simulator.