The recent emergence of anion-pi interactions has added a new dimension to supramolecular chemistry of anions. Yet, after a decade since its inception, actual mechanisms of anion-pi interactions remain highly debated. To elicit a complete and accurate understanding of how different anions interact with pi-electron-deficient 1,4,5,8-naphthalenediimides (NDIs) under different conditions, we have extensively studied these interactions using powerful experimental techniques. Herein, we demonstrate that, depending on the electron donating abilities (Lewis basicity) of anions and electron accepting abilities (pi-acidity) of NDIs, modes of anion-NDI interactions vary from extremely weak non-chromogenic anion-pi interactions to chromogenic anion induced charge transfer (CT) and electron transfer (ET) phenomena. In aprotic solvents, electron-donating abilities of anions generally follow their Lewis basicity order, whereas pi-acidity of NDIs can be fine tuned by installing different electron-rich and electron deficient substituents. While strongly Lewis basic anions (OH- and F-) undergo thermal ET with most NDIs, generating NDI center dot- radical anions and NDI2- dianions in aprotic solvents, weaker Lewis bases (AcO-, H2PO4-, Cl-, etc) often require the photoexcitation of moderately pi-acidic NDIs to generate the corresponding NDI center dot- radical anions via photoinduced ET (PET). Poorly Lewis basic I- does not participate in thermal ET or PET with most NDIs (except with strongly pi-acidic core substituted dicyano-NDI) but forms anion/NDI CT or anion-pi complexes. We have looked for experimental evidence that could indicate alternative mechanisms, such as a Meisenheimer complex or CH center dot center dot center dot anion hydrogen bond formation, but none was found to support these possibilities.