The 1980's and 1990's produced a variety of schools that can be put under the umbrella of the label "Computer Based Learning" (CBL). Frequently based on constructivist and cognitivist learning theories, these environments focussed on teaching both abstract and domain-specific problem solving. Preferred technologies were micro-worlds (computer environments were learners could explore and build), simulations (computer environments where learner can play with parameters of dynamic systems) and hypertext.