One of the most used theories for the sampling of materials for physical, chemical or biological testing is the theory developed by Pierre Gy. After a number of scientific publications, including several in the French language (e.g. Gy, 1953, 1964, 1975), he made –in 1979– his entire new theory available to the worldwide sampling community in a book (Gy, 1979) written in English. This book contains a complete description of Gy’s sampling theory. Later, Gy has made several refinements, but the essential character of the theory has always remained the same as the theory described in his 1979 book.
The impact of this book (and the entire theory of Gy) has been significant; even nowadays this book is regarded as the number one source of sampling-related information for engineers and process operators. Even though the practical impact and the scientific value of this work are unquestionably strong, several critical points of discussion need to be mentioned here, because the development of new technologies, recent experimental results and novel insights show that parts of Gy’s theory need to be updated or revised.