Flow injection analysis (FIA), was introduced in 1975 by Ruzicka and Hansen, The first generation of FIA technology, termed flow injection (FI), was inspired by the AutoAnalyzer technique invented by Skeggs in early 1950s. While Skeggs' AutoAnalyzer uses air segmentation to separate a flowing stream into numerous discrete segments to establish a long train of individual samples moving through a flow channel, FIA systems separate each sample from subsequent sample with a carrier reagent. While the AutoAnalyzer mixes sample homogeneously with reagents, in all FIA techniques sample and reagents are merged to form a concentration gradient that yields analysis results.
FIA methods can be used for both fast reactions as well as slow reactions. For slow reactions, a heater is often utilized.   The reaction does not need to reach completion since all samples and standards are given the same period to react.  For typical assays commonly measured with FIA (e.g., nitrite, nitrate, ammonia, phosphate) it is not uncommon to have a throughput of 60-120 samples per hour.
FIA methods are limited by the amount of time necessary to obtain a measurable signal since travel time through the tubing tends to broaden peaks to the point where samples can merge with each other. As a general rule, FIA methods should not be used if an adequate signal cannot be obtained within two minutes, and preferably less than one. Reactions that need longer reaction times should be segmented. However, considering the number of FIA publications and wide  variety of uses of FIA for serial assays, the "one minute" time limitation does not seem to be a serious limitation for most real life assays. Yet, assays based on slow chemical reactions have to be carried either in stopped flow mode ( SIA) or by segmenting the flow. 
OI Analytical, in its gas diffusion amperometric total cyanide method, uses a segmented flow injection analysis technique that allows reaction times of up to 10 minutes by flow injection analysis.
Technicon experimented with FIA long before it was championed by Ruzicka and Hansen. Andres Ferrari reported that analysis was possible without bubbles if flow rates were increased and tubing diameters decreased. In fact, Skegg's first attempts at the auto analyzer did not segment. Technicon chose to not pursue FIA because it increased reagent consumption and the cost of analysis.
The second generation of the FIA technique, called sequential injection analysis (SIA), was conceived in 1990 by Ruzicka and Marshal,  and has been further developed and miniaturized over the course of the following decade. It uses flow programming instead of the continuous flow regime (as used by CFA and FIA), that allows the flow rate and flow direction to be tailored to the need of individual steps of analytical protocol. Reactants are mixed by flow reversals and a measurement is carried out while the reaction mixture is arrested within the detector by stopping the flow. Microminiaturized chromatography is carried out on microcolumns that are automatically renewed by microfluidic manipulations. The discrete pumping and metering of microliter sample and reagent volumes used in SI only generates waste per each sample injection. The enormous volume of FI and SI literature documents the versatility of FI and SI and their usefulness for routine assays (in soil, water, environmental, biochemical and biotechnological assays) has demonstrated their potential to be used as a versatile research tool.