Paris once had a well-functioning system of tramways (aka streetcars), but these were replaced by buses in the 1930s under pressure from the motor vehicle manufacturers and tire companies.
The last of the old tram lines was discontinued on March 14, 1937. After that, Paris had no trams for over sixty-nine years, until the first section of the new T-3 (now the T-3a) went into operation on Saturday, December 16, 2006.
On the first weekend 120,000 people tried out the new tramway. One of my sons was among them, because the new tramway went right by his front door in the 13th arrondissement, on the southern edge of Paris.

Tram T-3a in the 13th arrondissement
These new tramways bear the number three because there were already two such tramways, the T-1 and the T-2, operating in the northern and western suburbs.
The tramways T-3a and T-3b now go along the boulevards at the southern, eastern and northern edges of Paris. While building the tracks and stations, they took the opportunity to widen sidewalks, plant trees, build new cycling paths and install an attractive new street lighting system, so as to upgrade these areas which previously were more like motorized jungles rather than habitable urban neighborhoods.

Tram T-3a at Porte de Vincennes
At first the intention was to have one long tramway line going all around the edges of the city of Paris, but now it has been divided into two separate lines, the T-3a (the ‘southern arc’) and the T-3b (the ‘northern arc’), which come together at Porte de Vincennes. The division into two arcs is intended to ensure the flexibility and dependability of the entire system.

Tram stop at Porte de Vincennes
At Porte de Vincennes, passengers can change from one arc to the other and also to the Métro line number 1 to get into the city center.
Together the T-3a and the T-3b now go roughly two-thirds of the way around the circumference of the city, from Pont du Garigliano to Porte d’Asnières. A further extension to Porte Dauphine is under construction and is scheduled to open in 2023, but it is doubtful that the tram will ever completely encircle the city because the rich people in the 16th and 17th arrondissements are opposed to it. Like most rich people, they are opposed to anything that they fear might get in the way of their sacred automobiles.

Tramway T-3b at Porte de la Chapelle
For non-motorists, a welcome and popular by-product of the new tramway lines is that the amount of space devoted to motor traffic on the boulevards is reduced (typically from six lanes to four) and the remaining motor traffic is calmed by giving the trams automatic priority at all street crossings.
My photos in this post are from 2007 and 2013. I revised the text in 2019.
See also: Public transport in Lyon.
Gotta love trams and good to see them making a comeback of sorts.
I feel trams bring a calmness to the city with its slow speed.