The absence of a schema in NoSQL databases can disorient tradi- tional database specialists and can make the design activity in this context a leap of faith. However, we show in this paper that an effective design methodology for NoSQL systems supporting the scalability, performance, and consistency of next-generation Web applications can be indeed devised. The approach is based on NoAM (NoSQL Abstract Model), a novel abstract data model for NoSQL databases, which is used to specify a system-independent representation of the application data. This intermediate representation can be then implemented in target NoSQL databases, taking into account their specific features.