As with all 'revolutions' there is not just ONE cause - historians talk about a range of different factors that all helped to generate revolution.
It helps to divide these factors into:

LONG TERM
SHORT TERM AND
TRIGGERS


LONG TERM (i.e. background factors that had been there for a long time).
Essentially this means all the things that were wrong with the Ancien Regime.

One fundamental cause was the rigid social structure of French society during the ancien regime. French society was divided into three Estates or Orders. The First Estate consisted of the clergy and the Second Estate the nobility. Together, these two Estates accounted for approximately 500,000 individuals. At the bottom of this hierarchy was the vast Third Estate which basically meant everybody else, or about 25 million people. This social structure was based on custom and tradition, but more important, it was also based on inequalities.

The First Estate - The Clergy.
From the outset, the clergy was established as a privileged Estate. The French Catholic Church maintained a wide scope of powers - it literally constituted a state within a state and it had sustained this position for more than 800 years. The clergy was divided into the lower and upper clergy. Members of the lower clergy were usually humble, poorly-paid and overworked village priests. As a group, they resented the wealth and arrogance of the upper clergy. The bishops and abbots filled the ranks of the upper clergy, men who regarded their office as a way of securing a larger income and the landed property that went with it. Most of the upper clergy sold their offices to subordinates, kept the revenue, and lived in Paris or at the seat of royal government at Versailles. They also owned 10-15% of all the land in France. This land, of course, was all held tax-free.

The Second Estate - The Nobility
Like the clergy, the nobility represented another privileged Estate. The nobility held the highest positions in the Church, the army and the government. They were virtually exempt from paying taxes of any kind. They collected rent from the peasant population who lived on their lands. They also collected an extraordinary amount of customary dues from the peasantry. There were labour dues (the corvee), as well as dues on salt, cloth, bread, wine and the use mills, granaries, presses and ovens. Collectively, the nobility owned about 30% of the land. By the 18th century, they were also becoming involved in banking, finance, shipping, insurance and manufacturing.

There were, like the clergy, two levels of the nobility (c.350,000 individuals in total). The Nobility of the Sword carried the most prestige. The served their King at his court in Versailles. Many members of this order were of ancient lineage - their family history could be traced back hundreds of years. But there were also members of this estate who were relative newcomers. The Nobility of the Robe also had prestige but much less than did the Nobility of the Sword. Numerous members of the Nobility of the Robe had been created by the monarchy in the past. French kings needed money so it seemed logical to offer position and status to those men who were willing to pay enough money for it.

The Third Estate
This estate consisted of every one who was not a member of either the First or Second Estates. Totaling approximately 25 million souls, the Third Estate was composed of the bourgeoisie, the peasantry and the urban artisans. As a class, the bourgeoisie - merchants, manufacturers, bankers, doctors, lawyers, intellectuals - had wealth. In some cases, enormous wealth. But, wealth in the ancien regime did not mean status or privilege. Wealth was nothing without status. The bourgeoisie were influenced by the nobility and tried to improve their status by becoming land owners themselves.

The peasantry consisted of at least twenty-one million individuals during the 18th century. Their standard of living was perhaps better than the European peasantry in general. However, the French peasant continued to live in utmost poverty. Peasants were victimized by heavy taxation - taxes were necessary to pay for the costs of war. So, the peasants paid taxes to the king, taxes to the church, taxes and dues to the lord of the manor, as well as numerous indirect taxes on wine, salt, and bread. Furthermore, the peasants also owed their lord a labour obligation. And throughout the 18th century, the price of rent was always increasing, as did the duties levied on goods sold in markets and fairs.

These, then, are the social causes that acted as a breeding ground for the grievances and passions the Revolution would unleash. But there are a few other causes, equally important, that are also worth our attention.

Royal Absolutism
Eighteenth century France was, in theory, an absolute monarchy. France had no Parliament. France did have an Estates General composed of representatives from each of the Three Estates but the last time the Estates General had been convened was in 1614!

Furthermore, France had no single, unified system of law. Each region determined its own laws based on the rule of the local Parlement.

Law
There were thirteen distinct regions in France before 1789 and each was under the jurisdiction of a Parlement. Each Parlement contained between fifty and 130 members. They were the local judges and legal elites. They tried cases for theft, murder, forgery, sedition and libel. They also served as public censors and sometimes were responsible for fixing the price of bread.
They were hated by almost everyone, including the king. Of course, the king also had his royal lackeys, the intendents. The intendents were even more hated than the Parlement - they became known for their habit of arbitrary taxation and arrest of the peasantry. Such a situation made for the inefficient operation of Europe's largest and strongest country.

SHORT TERM.

The Enlightenment
In the 18th century philosophical ideas associated with a period called the Enlightenment attacked the established order together with authority of any kind. These ideas helped to produce a revolutionary mentality - discontent with the existing state of things.

The American Revolution
There is little doubt that the American Revolution of the 1770s and the formation of a republic in the 1780s served as a profound example to all European observers. Hundreds of books, pamphlets and public lectures analyzed, romanticized and criticized the American rebellion against Great Britain. The American example served as a great lesson - tyranny could be challenged and new governments could be constructed. It got people into the habit of thinking more concretely about political questions, and made them more critical of their own governments and society. It brought written constitutions and declarations of rights into the realm of the possible.

The failings of Louis XVI and Marie Antoinette.
Finances
By 1789, France was bankrupt. The country could no longer pay its debts. By 1789, France was still paying off debts incurred by the wars of Louis XIV, that is, wars of the late 17th and early 18th century. Furthermore, a number of social groups and institutions did not pay taxes of any kind. Many universities were exempt from taxation as were the thirteen Parlements, cities like Paris, the Church and the clergy, the aristocracy and numerous members of the bourgeoisie. And of course, it was simply brilliant planning to continue to tax the peasants - peasants who, having nothing to contribute were, over the course of the century, forced to contribute even more.

Economic and social problems:
By 1789, the plight of the French peasant was obvious. Taxes and rent were increased. Peasants continued to use antiquated methods of agriculture. The price of bread soared and overall, prices continued to rise at a quicker rate than wages. To make matters worse, there was the poor harvest of 1788/89.
The urban workers or artisans, as a group, consisted of all journeymen, factory workers and wage earners. The urban poor also lived in poverty, a poverty that was intensified by 1789. By that time, wages had increased by 22% while the cost of living increased 62%.
By 1789, the bourgeoisie controlled 20% of all the land. They were upwardly mobile, but they felt frustrated and blocked by the aristocracy, an aristocracy whose only interest was that everyone maintain their place in society.
By 1789, the bourgeoisie had numerous grievances they wished addressed. They wanted all Church, army and government positions open to men of talent and merit. They sought a Parliament that would make all the laws for the nation. They wanted a constitution that would limit the king's powers. They also wanted fair trials, religious toleration and vast administrative reforms.

TRIGGERS.

The calling of the Estates General.
Louis' handling of the three estates.