A synesthete might inform you that McDonald’s is "all wrong", as obviously their large letter M has the completely wrong color. While synesthesia is now well accepted as a ‘real’ phenomenon, what underlies the highly specific and consistent additional sensations is topic of debate. What sets synesthetic mechanisms apart from those involved in ‘normal’ associations? In this presentation, we first discuss the possible neurobiological underpinnings. A review study has shown six brain regions related to synesthesia. Furthermore, results from structural as well as functional connectivity studies show hyperconnectivity in the synthete’s brain. Second, the behavioral characteristics that set synesthetes apart from non-synesthetes are discussed. One problem in obtaining a clear model of synesthesia is that currently, most studies are performed on particular types of synesthesia (in particular colored letters/numbers). We present rare cases of synesthesia (taste/smell with sounds) and examine how well their characteristics fit with the traditionally presented model of synesthesia.