A comparison is presented of the main approaches to second-law analysis (SLA) reported in the literature. Five SLAs are considered: exergy, physical-exergy, exergy-consumption, negentropy and entropy analyses. The application of the SLAs is illustrated through an example. In addition, several significant implications of SLA are examined in the fields of environmental impact and economics. The presentation herein of the approaches to and implications of SLA is expected to enhance understanding of SLA and improve its usefulness.