This text is a slightly edited version of lecture notes for a course I gave at ETH, during the Summer term 2001, to undergraduate Mathematics and Physics students. It covers a few selected topics from perturbation theory at an introductory level. Only certain results are proved, and for some of the most important theorems, sketches of the proofs are provided. Contents: Chapter 1 - Introduction and Examples Chapter 2 - Bifurcations and Unfolding Chapter 3 - Regular Perturbation Theory Chapter 4 - Singular Perturbation Theory