How to deal with students who didn't attend the exams? My course require students to take 2 mid-term exams and one final exam. What should I do if some students didn't attend the first mid-term exam or the second one. Should I mark them Zero or repeat the exam for them? What is the best strategy in this regard?

Update: Although I gave them instructions from the beginning of the course. Some of them said that they were absent when I gave those instructions. Some of them said that they were sick and some said that they had an appointment with a dentist at same time of the exam, for example!
I will talk from my own experience:


Being absent in the first class, is by no means a good excuse, back in my student days (before ubiquitous email information and professor's webpages) if you missed the first class, you would at least ask the sensible question of when the hell the midterm exam is.
As a student, their prime responsibility is to, well.. study! And while I understand that some of them would have conflicting issues if they have to work (which are usually solved if you have a nice enough boss), other scheduling issues like an appointment with the dentist (that can be done any other day) are also not a valid excuse.
Being a teacher of a 50+ student class, has a high likelihood that at least one of them may be sick the day of the exam, in those cases (hell, I've had teachers that were sick on the mid term's day), I agree that there should be some kind of Doctor's certificate around. 


If you think the absence is somewhat justified, you can always make them retake the exam.

In my school, most teachers would make the student do this over finals.