Despite the generalized use of Information and Communication Technologies (ICT) in teaching, their educational applications have not yet been standardized: a general consensus does not exist on how ICT can be applied to teaching nor on how educational software must be constructed. In this chapter, it is argued in favor of educational software construction being guided by a didactic problematique. In this framework we consider as a promising software category mindtools and, in particular, the so-called open microworlds. Their design must be guided by a number of principles: the tool logique, the multiple interface and the multiple representations principles. In this chapter, a detailed critique of these principles is also presented.