Grammar is an important aspect of teaching a second language. Since Turkish and English do not have one-to-one correspondence in terms of the present perfect and the present perfect continuous tenses, it creates difficulties for both Turkish teachers and Turkish students in teaching and using these two tenses. This study was conducted to determine the types of errors which Turkish students commit when using the present perfect tenses. It also investigates the sources of these errors and to what extent they are systematic. In addition, it explores the differences in the type of errors depending on the task which the students are required to complete, as well as finding out whether the teachers’ opinions reflect the results that are obtained from the students’ papers. Finally, suggestions on the materials that would be more suitable to Turkish students are prepared in the light o f the teachers’ opinions. The subjects were 120 students and nine teachers from the Department of Basic English at the Middle East Technical University. There are three levels of classes studying during the spring semester at METU: Pre-Intermediate, Intermediate and UpperIntermediate. From each level, three classes and the teachers of these classes were chosen for the study. There were 42 students from the Pre-Intermediate level, 40 from the Intermediate level, and 38 from the Upper-Intermediate level. The students were given a test devised by the researcher which consisted of five sections, graded from the most form-oriented tasks to the least. The first section was composed of fill-in-theblank questions; the second section was a sentence completion task; the third section involved the translation of a dialog between a native Turk and an American tourist mediated by an interpreter; the fourth section was a picture description and the last section was a paragraph writing task. While the students were answering the questions, their teachers were given written interviews in which they expressed their opinions about the difficulties that their students face in the present perfect tenses and the effectiveness of the materials that were being used in class to teach these two tenses. Data analysis involved making a record of students’ errors and classifying them into groups, and then, comparing these results with the opinions of the teachers. The results of this study indicated that Turkish students tend to make direct translations from Turkish into English, using the present continuous tense instead of the present perfect continuous tense, and the simple present or the simple past tense instead of the present perfect tense, caused by the differences in the adverb usages and verb types of the two languages. Additionally, the t)qjes of errors they committed showed consistency except in the function-oriented tasks, where contextual match was the most common error type Finally, the opinions of the teachers reflected the results obtained from the study. Considering these issues, the researcher suggested that consciousness-raising tasks and interpretation tasks be used along with the class materials and the native language be used to compare and contrast the two languages when necessary.