There are numerous legacies of ancient Greece that continue to affect the world today. You could conceivably choose to focus on topics such as architecture, athletics, art, drama, language, philosophy, literature, or academics. However, arguably the most indelible Greek development that continues to impact the world today is democracy.
It should be noted that democracy was not practiced in all of ancient Greece. Nor was it in practice for very long. Democracy, or rule by the people, was really only something that occurred in the city of Athens. Furthermore, it only lasted from the sixth century BCE to the fourth century BCE. Even then, Athenian democracy still experienced periodic interruptions and changes until it was ultimately suppressed by Alexander the Great in 322 BCE.
Democracy, as it was practiced in Athens, involved direct participation from adult male citizens who had received military training. This never rose higher than twenty percent of Athenians. Nevertheless, the idea that people can have a direct say in the operations of their government continues to exist in many societies around the world. Unlike ancient Athens, most modern democracies operate as representative democracies, where voters elect leaders to represent their interests. However, the notion that the ultimate source of power resides in the people themselves has endured.
Like in democratic Athens, the idea that all laws are publicly accessible and enforced uniformly continues to influence most modern democracies. To achieve this, it is necessary to have a written code of laws, like a constitution. Athens had one of these, possibly written by Aristotle himself, that compiled all the laws of the Athenian polis. A constitution clearly explains the structure of government and how laws are implemented and enforced. No modern democracy functions without a constitution of its own.
Athenian democracy stressed that all enfranchised citizens, even those in power, were to be beholden to the same laws as other citizens. Most modern democracies continue to hold on to this ideal, even if it is not always applied in practice.