The Qur’ān says that a person’s salvation in the Hereafter depends on the spiritual heart, not the physical heart that pumps blood. Hence, those who bring a pure heart ( qalbun sal ī m) on Judgment Day will enter Paradise. As for this earthly life, the Prophet Muhammad ﷺ said a pure heart is a prerequisite for a healthy body. The spiritual heart is the essence of our inner self, a repository of faith, God-consciousness, morality, and intentions. The heart is the master who orders the limbs to do its bidding. Interestingly, other faiths and cultural traditions also recognize a spiritual or inner heart. Now science is joining believers’ ranks, calling the heart a place of perception, emotion, and cognition. A heart has its own “brain” and a “magnetic field” that affects people around it. Like its physical counterpart, the spiritual heart can become diseased and cause mortal harm. This book discusses the causes of the heart’s major spiritual diseases and their cures.