π-Conjugated molecules with an even number of π-electrons usually have a closed-shell ground state. However, recent studies have demonstrated that a certain type of molecules could show open-shell singlet ground state and display diradical-like (diradicaloid) behavior. Their electronic structure can be understood in terms of the "diradical character" and "aromaticity" concepts. They display very different electronic properties from traditional closed-shell π-conjugated molecules and could be used as next-generation molecular materials.This book provides a comprehensive review on the chemistry, physics, and material applications of open-shell singlet diradicaloids. Particularly, it elaborates the fundamental structure-diradical character-electronic property relationships both theoretically and experimentally. The book has been written by leading scientists in the field from Japan, Germany, Spain, Italy, China, and Singapore.