The course was originally offered to second year undergraduate Materials Science students at the Swiss Institute of Technology during the author’s sabbatical leave in French. The notes were developed with the following objectives in mind. First they must be complete and rigorous. At any time, a student should be able to trace back the development of an equation. Furthermore, by going through all the derivations, the student would understand the limitations and assumptions behind every model. Finally, the rigor adopted in the coverage of the subject should serve as an example to the students of the rigor expected from them in solving other scientific or engineering problems. This last aspect is often forgotten.