By all accounts all the works of early Greek philosophers were lost forever. Everything we know about their teachings we owe testimonies which have been preserved in the writings of later thinkers. Today it is not possible to seriously consider the earliest Greek philosophy without consulting studies dealing with testimonies about that philosophy. We will here devote our attention to these testimonies, to their importance, to their classification, history of their researches, as well as state of the research in our time. When it comes to classification of genres of testimonies, we will illustrate the most striking representatives of each of them. Central role in the history of this research played a Hermann Diels. We will highlight the importance and achievements of his research, to say something about his predecessors and successors, as well as today's scrutiny of its grandiose works Doxographi Graeci and Die Fragmente der Vorsokratiker.