Recent research in applied linguistics emphasizes the significance of learners' errors in second language learning. In this article, major types of errors in second language learning are first briefly mentioned. This is followed by tracing the sources of second language learning errors to both interlingual and intralingual or developmental factors. While interlingual errors are caused mainly by mother tongue interference, intralingual or developmental errors originate in the following factors: simplification, overgeneralization, hypercorrection, faulty teaching, fossilization, avoidance, inadequate learning, and false concepts hypothesized. The article concludes with some general guidelines for teachers in correcting errors in second language learning.