Reflectometry using Global Navigation Satellite System (GNSS-R) signals has become an attractive tool for Earth remote sensing. Back to 1993, it was proposed as an alternative to the conventional spaceborne ocean altimetry to estimate the sea surface height over a large field-of-view. After Doppler compensation, the cross-correlation of the received scattered signal with either a locally generated replica [conventional GNSS-R (cGNSS-R)] or either the direct signal [interferometric GNSS-R (iGNSS-R)] is called the waveform. This paper provides a critical review of the cross-correlation waveform model, addressing issues such as the correlation characteristics, the bandwidth, the observation geometry, and the thermal and speckle noises. It provides a comprehensive and systematic overview of the impact of all these effects on the GNSS-R observables, which is important to properly define future GNSS-R instrument and mission concepts.