Chapter 2: contains a summary of the main properties and the basic theories of the IGM.
Chapter 3: contains a summary of the main properties and the basic theories of the MUVB.
Chapter 4: discusses why and how to establish a semi-analytical photoionization model.
Chapter 5: presentation of some applications to metal absorption line systems in HS1700+6416. Chapter 6: we investigate proximity effect on metal elements.
