In the past ten to fifteen plus years, the concept of major seawater ion toxicity (MSIT) has gone from its infancy to near maturity as a general concern with membrane concentrates. Concerns with MSIT are relevant only in those situations where membrane concentrate disposal is through discharge to waters that require compliance with state and/or Federal toxicity standards. In those cases where membrane concentrate disposal is through deep well injection or some other means that does not require toxicity testing, MSIT is generally not an issue or concern. Although the concept had been identified and studied in relation to other industries and toxicity analysis, the earliest applications of MSIT evaluation in relation to membrane concentrates were formally conducted by the Florida Department of Environmental Protection (FDEP) in the early 1990s. This effort culminated in the FDEP publication of a 1995 guidance document entitled "Protocols for Determining Major-Seawater-Ion Toxicity in Membrane-Technology Water-Treatment Concentrate". The guidance was based on evaluations of concentrate from a variety of facilities around the state ranging from low TDS concentrates from membrane softening facilities up through higher salinity (20 to 25 ppt) concentrates from brackish water RO facilities. Since the publication of the 1995 guidance, evaluation of MSIT in membrane concentrates has become more and more popular in Florida. The concept has gained acceptance in both the regulatory arena as well as the regulated facilities. In fact, in the last several years legislation has been introduced in Florida to codify exemptions and specific regulatory mechanisms to streamline the application of MSIT evaluation in review of membrane concentrate surface discharge. Due to increasing popularity, testing techniques and permit evaluations for MSIT have been under way in several states in the southeast and elsewhere. The US Environmental Protection Agency has been reviewing the issue on a case by case basis in the general context of Toxicity Identification Evaluation techniques and determinations. In general, the observed toxicity that results from MSIT is short-lived in the environment, with one to one dilution often more than enough to erase any toxicity that may be attributable to MSIT. Includes tables.