Deferral structures share one rule that explains all of them: the taxpayer must never have the right to receive the money. Every technical requirement in a like-kind exchange, an opportunity zone investment, or an installment sale is a device for keeping cash out of the seller's control while the substitute investment is arranged. This article explains how that principle operates, why a qualified intermediary exists and what happens when the agreement gives the taxpayer even a theoretical right to the funds, how the forty-five-day identification and one-hundred-eighty-day exchange periods work and why they are absolute, what boot is and how it is netted, and why depreciation recapture and basis carryover mean deferral is not forgiveness. It then covers opportunity zone investments and their very different architecture, installment reporting, involuntary conversions, and the substance-over-form doctrines that police the whole area.