The word orchestra originally comes from the ancient Greeks, where it meant the space in front of the stage where a chorus sang and danced.  Orchestra began to be applied to a group of musicians during the late 17th and early 18th centuries.  Both the meaning of the word and the orchestra itself have evolved over time to be what we now mean today - "an organised body of bowed strings with more than one player to a part, to which may be added any number of wind and percussion instruments" (Grove Dictionary of Music and Musicians). 

The orchestra as a particular group of musicians first became prominent in the Baroque period (approx AD 1650 - 1710) and mainly consisted of string instruments with a continuo keyboard instrument, such as the harpsichord, and some wind instruments were added when required.  At this time, the orchestra was mainly directed by the person playing the keyboard continuo instrument and later by the principal first violin. 

• Pairs of wind instruments became an established part of the orchestra during the Classical period (approx AD 1750 - 1810)
• Through the Romantic period (approx AD 1810 - 1914), the number of wind and brass instruments used was expanded as composers used a wider variety of sounds in their orchestral music
• The changes in the instruments used as a standard part of an orchestra are clearly linked to the technical advancements made by instrument makers
• During the twentieth century the number and range of percussion instruments used was expanded
• The end of the twentieth century saw the development of orchestra education and community work as a core part of the orchestra's activity. 

The role of the chamber orchestra developed in the early twentieth century partly as a result of a reaction to the excesses of the Romantic era, but also to the idea of playing music with the forces it was written for at the time.  The performance practice movement changed the way that we heard music by particular composers.  For example, J.S. Bach's choral music was originally written for a small choir, so imagine if you had only ever heard it sung by a large choral society of about 100 singers and then heard it sung by a choir of 10 - 15; it can almost sound like a different piece of music. 

These explorations of how music was performed at the time it was written have changed the way many orchestras perform, so when playing music from the Classical period many large symphony orchestras often reduce the size of the orchestra.  Some chamber orchestras took the idea of performance practice further and used originals or copies of instruments of the period when a piece of music was written.  The sound of these 'period orchestras' is often very different to the modern day symphony orchestra and can give a different perspective on the music itself.  It can be easy to forget that composer's like Beethoven were stretching the abilities of the musicians and their instruments, which gives the music a different aspect to when we hear it on modern instruments.

The orchestra is continually evolving and although the instruments of the orchestra have become standardised, composers and musicians are constantly exploring new sounds and ways of playing instruments, as well as the range of instruments from other cultures that have become widely available during the twentieth century.  Also, as orchestra education and community programmes grow, a new genre of music and way of music making is developing with them.  Weaving all of these experiments into the twenty first century orchestra is a challenge but one orchestras' are embracing.