Vacuum technology has advanced considerably over the last 25 years and very low pressures are now routinely obtainable. Firstly, we need to remind ourselves of the units of pressure: The SI unit of pressure is the Pascal ( 1 Pa = 1 N m -2 ) Normal atmospheric pressure ( 1 atm.) is 101325 Pa or 1013 mbar ( 1 bar = 10 5 Pa ). An older unit of pressure is the Torr ( 1 Torr = 1 mmHg ). One atmosphere is ca. 760 Torr ( i.e. 1 Torr = 133.3 Pa ). While the mbar is often used as a unit of pressure for describing the level of vacuum, the most commonly employed unit is still the Torr (the SI unit , the Pa, is almost never used). Classification of the degree of vacuum is hardly an exact science - it much depends upon who you are talking to - but as a rough guideline: 0 1 Rough (low) vacuum: 1 - 10-3 Torr Medium vacuum: 10-3 - 10-5 Torr High vacuum (HV): 10-6 - 10-8 Torr Ultrahigh vacuum (UHV): < 10-9 Torr Virtually all surface studies are carried out under UHV conditions - the question is why ? This is the question that we will address in Section 4.2 .