Engineering disciplines are characterised by the use of mature knowledge by means of which they can achieve predictable results. Unfortunately, the type of knowledge used in software engineering can be considered to be of a relatively low maturity, and developers are guided by reasoning based on intuition, fashion or market-speak rather than by facts or undisputed statements proper to an engineering discipline. This paper analyses the maturity level of the knowledge about testing techniques by examining existing empirical studies about these techniques. For this pu1pose, three categories of knowledge of increasing maturity have been presented and the results of these empirical studies have been placed in these three categories.