Krashen's theories of second language acquisition consist of five main hypotheses: the Acquisition-Learning Hypothesis, the Monitor Hypothesis, the Natural Order Hypothesis, the Input Hypothesis, and the Affective Filter Hypothesis. These hypotheses emphasize that acquisition is more important than learning, acquisition occurs through comprehensible input, and affective factors like anxiety and motivation can help or hinder acquisition. The Natural Approach developed by Krashen focuses on providing meaningful input through activities and realia to develop communicative skills in a low-anxiety environment.