The Christian life is not a playground, but a battleground. One of the fiercest foes in this battle dwells within our own hearts: the enemy of indwelling sin. The Scriptures command us to "put sin to death." This is what pastors and theologians of another generation called the "mortification" of sin. But how do we mortify sin? And what role does the gospel play in this effort to apply lethal force against sin? How can we avoid falling into legalism while still maintaining a passion for holiness? And what kinds of strategies actually work in the daily battle? These are questions I have personally wrestled with as both a Christian and a pastor. This book is my attempt to answer those questions and provide a biblical and practical guide for the war against sin.