Very few buildings (especially churches) were actually built in only one style. Massive, expensive structures often took centuries to complete, during which time tastes changed and plans were altered.
Classical (6th Century B.C. to 4th Century A.D.)
The Romans made use of certain Greek innovations, particularly architectural ideas. Classical orders are most easily recognized by their column capitals, with the least-ornate capital used on a building's ground level and the most ornate used on the top: Doric (a plain capital), Ionic (a capital with a scroll), and Corinthian (a capital with flowering acanthus leaves).
Although marble is traditionally associated with Roman architecture, Roman engineers could also do wonders with bricks or even simple concrete -- concrete seating made possible such enormous theaters as Rome's 2.5-hectare (6-acre), 45,000-seat Colosseum.
Roman architecture includes the sports stadium of the Colosseum (1st c. A.D.; see illustration), which perfectly displays the use of the classical orders; Hadrian's marvel of engineering, the temple of the Pantheon (1st c. A.D.); and the public Baths of Caracalla (3rd c. A.D.). Another good example is the Basilica of Constantine and Maxentius in the Roman Forum (4th c. A.D.).
Romanesque & Gothic (7th to 15th Centuries)
The Romanesque took its inspiration and rounded arches from ancient Rome (hence the name). The first major churches in Rome were built on the basilica plan of Roman law courts. Architects constructed large churches with wide aisles to accommodate the masses.
By the late 12th century, the development of the pointed arch and exterior flying buttress freed architecture from the heavy, thick walls of Romanesque structures and allowed ceilings to soar, walls to thin, and windows to proliferate in the Gothic style.
The great early basilicas, each at least partly altered in decor over the ages, include Santa Maria Maggiore, San Giovanni in Laterano, and San Paolo Fuori le Mure. Other less grand Romanesque churches include Santa Maria in Cosmedin and Santa Sabina.
Santa Maria Sopra Minerva is Rome's only Gothic church, all pointy arches and soaring ceilings.
Renaissance (15th to 17th Centuries)
As in painting, Renaissance architectural rules stressed proportion, order, classical inspiration, and mathematical precision to create unified, balanced structures.
Bramante (1444-1514) was the most mathematical and classically precise of the early High Renaissance architects. This is evident in his (much altered) plans for St. Peter's Basilica (his spiral staircase in the Vatican has survived untouched) and his jewel of perfect Renaissance architecture, the textbook Tempietto (1502; see illustration) at San Pietro in Montorio on the slopes of Rome's Janiculum Hill.
Renaissance man Michelangelo took up architecture later in life, designing the dome atop St. Peter's Basilica; the sloping approach, 12-pointed star courtyard, and trio of palace facades that together make up Piazza del Campidoglio atop the Capitoline Hill; and the facade of the Palazzo Farnese (1566), which was otherwise built by Antonio da Sangallo (1483-1546).
Baroque & Rococo (17th to 18th Centuries)
More than any other movement, the baroque aimed toward a seamless meshing of architecture and art. The stuccoes, sculptures, and paintings were all carefully designed to complement one another -- and the space itself -- to create a unified whole.
Though relatively sedate, St. Peter's facade by Carlo Maderno (ca. 1556-1629) and sweeping elliptical colonnade by Bernini make for one of Italy's most famous baroque assemblages. The two also collaborated (along with Borromini) on the Palazzo Barberini (1620-30s). The painter Pietro da Cortona designed the distinctive semicircular portico on Santa Maria della Pace (1656-57).
For the rococo -- more a decorative than architectural movement -- look no farther than the Spanish Steps (1726), by architect Francesco de Sanctis (1693-1740), or the Trevi Fountain (1762), by Nicola Salvi (1697-1751).
Neoclassical to Modern (18th to 20th Centuries)
As a backlash against the excesses of the baroque and rococo, by the middle of the 18th century, Italian architects began turning to the austere simplicity and grandeur of the Classical Age and inaugurated the neoclassical style. Neoclassicists reinterpreted ancient temples as buildings with massive colonnaded porticos.
From the 19th century through the 20th century, Italian architects constructed buildings in a variety of styles. Italy's take on the early-20th-century Art Nouveau movement was called Liberty style. Mussolini made a spirited attempt to bring back ancient Rome in what can only be called fascist architecture. Since then, Italy, like the rest of the world, has mostly erected concrete and glass skyscrapers.
Of the neoclassical, the Vittorio Emanuele Monument, which has been compared to a wedding cake and a Victorian typewriter, was Italy's main monument to reaching its Risorgimento goal of a unified Italy.
Liberty style never produced any surpassingly important buildings, although you can glimpse it occasionally in period storefronts.
Fascist architecture still infests all corners of Rome. You can see it at its, er, best in Rome's planned satellite community called EUR (including a multistory "square Colosseum" so funky it has been featured in many films and music videos) and the Stadio Olimpico complex.
The mid-20th century was dominated by Pier Luigi Nervi (1891-1979) and his reinforced concrete buildings, including the Palazzetto dello Sport stadium (1960).