The municipalities of Puerto Rico number seventy-eight incorporated towns and cities. Each municipality is led by a mayor and divided into barrios, though the latter are not vested with any political authority. Every municipality is governed by the Autonomous Municipalities Act of 1991, which establishes that every municipality must have an elected strong mayor with a municipal legislature as the form of government. Each legislature must be unicameral, with the number of members related to adequate representation of the total municipality's population. In contrast to other jurisdictions, both the mayors and the municipal legislators are elected on the same date and for the same term of office.
From a political and urban design perspective, several differences and similarities exist among municipalities of differing sizes. For instance, municipalities with 50,000 inhabitants or more are considered incorporated cities, while those with fewer than 50,000 are considered incorporated towns. Size affects the autonomy exercised by the jurisdiction: cities provide and manage their own services, while towns typically depend on nearby cities for certain services. All municipalities have a barrio called pueblo proper (English: 'town'), which typically is the site of that municipality's historic Spanish colonial settlement. With development over time, it typically has become that municipality's urban core. Municipalities with large populations, however, may have an urban core that consist of several barrios.
Other differences exist among the municipalities. Economic activity, for example, tends to be concentrated in the metropolitan areas surrounding the cities of San Juan, Ponce, Arecibo, Caguas, Mayaguez, Aguadilla, and Humacao, and most towns are commuter towns. Statistically, the municipality with the largest number of inhabitants is San Juan, with around 400,000, while Culebra is the smallest, with around 1,800. Arecibo is the largest in terms of geography, with around 125 mi2, and Cataño the smallest, with around 4.8 mi2.
Every municipality is composed of several barrios, except for Florida, which has only one barrio. The municipality of Ponce has the largest number of barrios, 31.
Every municipality (except San Juan) also has an urban area made up of one or more barrios. When the urban area is made up of only one barrio, it is called "Barrio Pueblo". Some urban areas are made up of multiple barrios: Ponce's urban area, for example, is made up of 12 barrios. All of San Juan's barrios are urban barrios, and the municipality of San Juan is composed of urban barrios only - thus, the entire municipality of San Juan consists of one large urban zone only.
Every municipality's urban zone is named by the same name as the municipality. For example, the municipality of Caguas has an urban zone called Caguas - just like the municipality. Some municipalities' urban zones are termed "pueblo" (town) while others are termed "ciudad" (city). The difference resides in the population of the municipality: if the municipality has an urban zone below 50,000 inhabitants, then its urban zone is called a pueblo. If a municipality has a population above 50,000 inhabitants in its urban zone, then its urban zone is called a ciudad.
In October 2009, a Puerto Rican legislator proposed a bill that would reduce the current 78 municipalities of Puerto Rico down to 20. The bill called for a referendum to take place on June 13, 2010, which would let the people decide on the matter. However, the bill never made into law.
It was not the first time that an attempt to consolidate municipalities had failed. In 1902 the Puerto Rico legislature, under pressure from the US.-appointed governor of Puerto Rico, passed a law consolidating the then-76 municipalities of Puerto Rico into 46. The law was repealed three years later.
With the serious fiscal crisis that emerged in the first half of 2010s, a new plan to consolidate municipalities was again circulated in the legislature in 2017 as a way to alleviate the government debt crisis, but even that plan was also ultimately defeated. In March 2019 Governor Ricardo Rosello created an initiative which would preserve the existing municipalities but create regional consolidation by sharing service overhead in the form of counties.
A legislator's 2009 proposal of Puerto Rico's future set of municipalities and their boundaries
^An Act: To amend Sections 1 and 2 of Act No. 100 of June 27, 1956 Act No. 81 of August 30, 1991: Autonomous Municipalities Act of the Commonwealth of Puerto Rico. In An Act: To amend Sections 1 and 2 of Act No. 100 of June 27, 1956, Act No. 66, 3rd Session of the 13th Legislature of Puerto Rico. April 14, 1998. Retrieved November 22, 2009.
^Ponce. Proyecto Salon Hogar. Map of Barrios of Ponce. (Map with fully urbanized barrios conglomerated and merged as "Zona Urbana". Barrio not labeled is named "Machuelo Abajo".) Retrieved November 30, 2009.