An expert is a person who has made all the mistakes that can be made in a very narrow field. (Niels Bohr, 1885 - 1962).
Traditionally, scientific papers and seminars tend to report only success, without mentioning how many mistakes and unsuccessful attempts were required to get there, and without recalling all what remains to be done when you have achieved your definition of “success”. In the past half century, CERN has built on average a linear accelerator every 18 years, accumulating a large amount of experience, defined in Bohr’s way, which now went into the construction of the new 160 MeV Linac4 that has just entered beam commissioning and has already accumulated its own record of “experience”. After an introduction on the CERN linacs and on the Linac4 project, this seminar will make an attempt at sharing this experience, starting from the physics design phase and going to mechanical design, integration, procurements, international collaborations, and finally down to the bitter end of beam commissioning that Linac4 is now facing. An analysis of faults and reliability of the present Linac2 will be presented to complete the picture.