In ATSC 3.0, a bootstrap is used to enable service discovery and signaling reception at low signal levels. The ATSC 3.0 bootstrap is described in this paper. The bootstrap signal consists of several specially-coded OFDM symbols. The first bootstrap symbol is used for detection, synchronization, and service discovery. The remaining symbols convey system parameters describing the associated physical layer frame structure, e.g., signal bandwidth, sampling rate. The originating sequence for bootstrap signaling is a combination of a Zadoff-Chu sequence and a pseudo-noise sequence in the frequency domain. Signaling information is conveyed through the use of cyclic shifts in the time domain. The coarse timing synchronization, symbol detection, and fractional frequency offset estimation are performed simultaneously in the time domain, while the integral frequency offset estimation and the transmission parameter signaling decoding are performed in the frequency domain. Superior detection and decoding performance are proved by simulation in typical use cases, and very low thresholds and capability in difficult channel models are enabled by the use of the bootstrap.