The money an employer withholds from a paycheck was never the employer's money. The statute says so expressly, and everything distinctive about payroll tax enforcement follows from that one sentence. This article explains why the withheld portion is held in trust, how the trust fund recovery penalty reaches individuals personally regardless of corporate form, what makes a person "responsible" and conduct "willful" in a sense that has almost nothing to do with intent to defraud, and why paying other creditors first is the single most common way business owners incur six-figure personal liability. It then covers deposit rules and the penalty that scales with lateness, the worker classification exposure that generates the underlying assessments, the interaction with bankruptcy and payment designation, and the criminal statute that sits behind the civil penalty.