Small micellar aggregates of some surfactants exhibit enormous growth in one dimension and form very long and flexible wormlike micelles. Depending on the nature of the surfactant, such micellar growth can be induced in different ways, for example by adding cosurfactants or salts. Above a system-dependent concentration of surfactant, these giant micelles are entangled to form a transient network, and exhibit viscoelastic behavior analogous to a flexible polymer solution. However, unlike polymers in solutions, wormlike micelles undergo breaking and recombination, and, therefore, exhibit complex rheological behavior. Information on the evolution of aggregate morphology can be obtained from rheological study. In this article formation of wormlike micelles and the evolution of rheological properties in different mixed surfactant systems is discussed. Besides, a brief overview on the salt-induced micellar growth in ionic surfactant systems and reverse micellar systems induced by adding certain polar additives has also been presented.