This chapter describes the modeling of nullor-based active devices from the circuit level of abstraction. After a brief overview on the nullor concept and its properties, the modeling of active devices not only at the voltage-mode but also at the current-mode and the mixed-mode of operation from two-port and four-terminal network point of view is described in some detail. An important view that permeates the chapter is that the nullor-based models are not too complex and they can be introduced in CAD tools. Furthermore, parasitic elements can easily be added in order to predict their impact on the final response of the circuit. Several examples using nullor-based models illustrate its use to calculate fully-symbolic small-signal characteristics of linear or linearized analog circuits.