In this Chapter, our goal is to describe how π-conjugated chromophores respond to an external electric field and establish the molecular basis for a large NLO response. To do so, we focus on molecular polarizabilities and derive the physical picture that links the chemical and geometric structure, ground-state and excited-state electronic properties, and NLO response. These connections allow us to establish the molecular design requirements that can lead to new generations of NLO materials. We begin by defining the derivative relationships between molecular polarizabilities and total energy in π-conjugated chromophores. We then describe the (simplified) sum-over-states (SOS) expressions for the molecular polarizabilities, which provide an understanding of the electronic states primarily involved in the NLO response. Finally, we show how simple electronic-structure approaches in combination with the SOS expressions can be exploited to link these electronic states to the molecular architecture.