Abstract: Objective Ambiguity resolution is a core issue in high precision GNSS carrier phase positioning,inclu-ding both ambiguity estimation and ambiguity validation.Ambiguity estimation fixes the float ambigu-ity to its integer value utilizing its precision information:an integer least-squares problem.Ambiguityvalidation evaluates the estimated ambiguity and whether it can be treated as the correct:aprobabilitystatistics problem.Incorrectly validated ambiguity can lead to wrong positioning results at the decime-ter or larger bias.Therefore,ambiguity validation will have a direct impact on the reliability of ambi-guity resolution and positioning results based on the theory of ambiguity validation.We introduce sev-eral widely used ambiguity validation methodologies.The advantages and disadvantages of each meth-od are compared and analyzed,from the perspective of the background theory and practicability.Atlast,we address the outlook for future research in ambiguity validation.