This place is absolutely breath-taking. I've never seen such a highly decorated church. It has to be one of the must sees on any trip to the French capital.
The Saint Chapelle was begun in , on the orders of Louis IX, later Saint Louis, in the 13th Century. Louis had acquired the Crown of Thorns which Christ wore, for France, and wanted somewhere suitable to store this sacred relic. The idea of Saint Chapelle was to built an enormous reliquary in the middle of the royal palace on the Ile de la Cite (this palace is now the law courts. Having the chapel in the royal palace reinforced the closeness between the French monarchy and God, hence the chapel being decorated with Fleur de Lys and sysmbols of Christ.
The chapel is located on the Ile de la Cite, which is on the Seine, right in the heart of Paris. It is a few minutes walk from Notre Damme. You can reach it by metro, line 4.
The entrance fee is between 5.5-6.1 Euros depending on whether you take a tour. The chapel is also included on the Museums and Monuments Pass which is a fantastic investment if you're planning on seeing a couple of sights. I imagine that a tour would be well worthwhile, but there are information cards in the chapel which you can read at your leisure.
The website claims that you can access the chapel if you are disabled but I can remember a spiral staircase connecting the upper and lower chapels so this may be worth asking about first.
You enter through the lower chapel. This is sligthly plainer and is dedicated to the Virgin Mary. The ceiling is painted blue with gold fleur de lys decorating it. There are also pillars, some of which are blue with gold fleur de lys (representing Louis) and some which are red with gold castles (representing Blanche of Castile, Louis' mother).
You then progress to the upper chapel. Be prepared, it will amaze you. It gives the appearance of every inch being decorated. The 'walls' are in fact stained glass windows, which give the appearance that the ceiling is floating. Each window tells a story from the Bible, and one tells the story of the Passion relics, right up to them being received by Louis (the royal connections with God are illustrated throughout). The ceiling is completly painted. There are statues of the Apostles as well. The reliquary still exists in the chapel although it now lies empty (the relics are now housed in Notre Damme). However, I don't think that this really detracts from the visit. The fact that this was all built so long ago (albeit with restoration), and that in the 13th century something so amazing could be built at all, will keep you in awe.