This paper reviews recent research activities on the applications and technical issues of WDM-PON. First, we describe two candidate applications of WDM-PON: one is a long-reach WDM-PON based on wavelength routing for metro/access integration, and the other is a short-reach WDM-PON for co-existence with current PON systems. We identify that the realization of colorless optical network units (ONUs) is an important technical issue for both applications, and an effective protection function is another important issue, especially for the long-reach WDM-PON. We introduce several 'colorless-ONU' approaches as well as introduce our recent research results to resolve the issues raised by some colorless-ONU approaches. In the second half of this paper, we describe two application: one is the wide-area optical/wireless hybrid access network based on long-reach WDM-PON with loop-back type colorless ONUs, and the other is the optical and wireless access based on short-reach WDM-PON coexisting with a current PON infrastructure with tunable-type colorless ONUs.