...One of the defining features of ALT cells is the prevalence of extrachromosomal telomeric repeat (ECTR) DNA. Here, we identify that ALT cells engage in two modes of telomere synthesis. Non-productive telomere synthesis occurs during the G2 phase of the cell cycle and is characterized by newly synthesized internal telomeric regions that are not retained in the subsequent G1, coinciding with an induction of ECTR DNA. Productive telomere synthesis occurs specifically during the transition from G2 to mitosis and is defined as the extension of the telomere termini. While many proteins associated with break-induced telomere synthesis function in both non-productive and productive telomere synthesis, POLH specifically promotes productive telomere lengthening and suppresses non-productive telomere synthesis. These findings delineate the mechanism and cell cycle regulation of ALT-mediated telomere synthesis and extension.