The Internet plays a significant role in the lives of young people today. Previous research points to advantages as well as disadvantages of the use of the Internet in a formal educational context. The aim of the current study was to investigate the influence of the use of the Internet on planning and instruction. Five upper secondary school teachers were interviewed face-to-face. Five recurrent themes were identified in the interview data: general opinions on and experience of the Internet, attitudes to teaching and learning, opinions on the use of the Internet as a planning and teaching resource, effects of the use of the Internet on students and teachers, and drawbacks of the use of the Internet in the school. It was found that the teachers think that the Internet is a valuable source of information and an important additional teaching tool. The Internet can e.g. motivate the students, make teaching more fun, and allows variation in teaching. Four major drawbacks of the use of the Internet were reported by the teachers, viz. students' cheating, unreliable information, technical problems, and students' extracurricular activities during lessons.