This paper is on consensus protocols for synchronous systems where processes can commit crash failures, omission failures or Byzantine failures. Is presents and revisits consensus protocols coping with such failures in an increasing order of difficulty.The paper can be seen as a short tutorial whose aim is to make the reader familiar with synchrony assumptions, different definitions of the consensus problem, and a hierarchy of process failure models. An important concern of the paper lies in simplicity. In addition to the survey flavor of the paper, several results that are presented are new, among which the ones concerning the omission failure model.