This　chapter　concerns　the　second　law　analysis　of　latent　heat　storage　using　phase　change　materials　(PCMs).　The　discussion　covers　the　PCMs　and　the　PCM-based　devices　and　systems.　The　analyses　on　the　PCMs　focused　on　the　irreversibilities　of　phase　change　processes　particularly　due　to　supercooling,　and　adding　thermally　conductive　particles　or　matrixes　to　enhance　the　heat　transfer　process.　The　analysis　of　PCM-based　devices　and　systems　include　three　aspects:　(a)　key　evaluation　parameters　for　assessing　the　performance　of　latent　heat　storage　devices,　and　the　use　of　such　parameters　to　optimize　geometric　designs　and　device　configurations;　(b)　approaches　for　enhancing　thermal　performance　of　latent　heat　storage　systems,　particularly　the　effectiveness　of　the　cascading　use　of　PCMs,　the　PCM　melting　temperature　range,　and　the　PCM　arrangement;　and　(c)　key　processes　that　give　rise　to　major　exergy　losses　through　an　exergy　balance　diagram.　©　2022　Elsevier　Inc.　All　rights　reserved