In-line　digital　holography　is　widely　applied　in　many　areas　due　to　its　simple　optical　path,　low　requirement　of　coherence　and　fully　utilization　of　camera＇s　bandwidth.　The　reconstruction　quality　of　in-line　digital　holography,　however,　suffers　from　the　superposition　of　twin-image　contamination,　which　can　be　solved　via　phase　retrieval　algorithm.　By　iteratively　applying　certain　constraints　on　an　estimate　of　an　object　in　real　and　reciprocal　domain,　the　real　image　is　well　seperated　from　the　unwanted　components.　The　recording　and　reconstruction　process　of　in-line　digital　holography　and　fundemental　phase　retrieval　methods　including　Gerchberg-Saxton　(GS),　error　reduction　(ER)　and　hybrid　input-output　(HIO)　algorithms　are　introduced.　Then,　the　phase　retrieval　algorithms　for　in-line　digital　holography　is　divided　into　four　categories,　i.e.,　constraints,　multiple　axially　recording,　wavelength　divergency　and　initial　guess.　The　constraints　and　experimental　results　of　these　phase　retrieval　algorithms　are　listed.　Finally,　a　prospect　is　given　for　the　possible　improvement　and　potential　applications　of　phase　retrieval　methods　in　in-line　digital　holography.