The purpose of this introductory module is to allow the student to become rather more familiar with the very powerful SI unitary system than is perhaps the case now.  Most will never have visited BIPM.org, IUPAC.org or maybe NPL.co.uk, to see how SI actually works; this may apply equally to those of us who teach and instruct too.  Although not short on detail, the present document is intended to be less heavy going than official documentation from metrology bodies, but it is certainly not short on detail and rigour in as far as what can be achieved within its 30 pages.  The material is approachable at the 16+ level or, with appropriate guidance, somewhat earlier, particularly for those motivated towards the physical sciences, engineering or medicine.  It should prove valuable during a programme of study.