Determinism is easy to state and hard to escape. If every event has causes, and your decisions are events, then your decisions have causes — which had causes of their own, running back through your upbringing, your genes, and the state of the universe long before you existed. Say that clearly and something uncomfortable follows: at the moment of choosing, could you have done otherwise?
The thesis comes in versions. Causal determinism holds that every event is necessitated by prior events plus natural law. Logical determinism argues from the truth of statements about the future. Theological determinism derives it from divine foreknowledge. They arrive at similar places by different roads, and it is worth noticing that determinism is not a claim about prediction — no one could actually compute the future — but about what is fixed, whether or not anyone knows it.
Three positions dominate the response. Hard determinists accept determinism and conclude that free will is an illusion, which forces a rethink of blame and desert. Libertarians (in the metaphysical, not political, sense) insist that we do have genuine alternative possibilities, and therefore that determinism must be false somewhere. Compatibilists — probably the largest camp among philosophers — argue the dispute rests on a bad definition: freedom was never about escaping causation, but about acting on your own desires and reasons without external coercion. A person choosing dessert freely and a person choosing at gunpoint are both fully caused, and the difference between them is exactly what "free" was always tracking.
Modern physics complicates the picture without settling it. Quantum mechanics appears to introduce genuine indeterminacy at small scales, which means the universe may not be deterministic in the strict classical sense. Even this is contested: on some interpretations of quantum mechanics, such as Bohmian mechanics and the many-worlds interpretation, the underlying dynamics remain fully deterministic. But this rescues free will less than people hope: random events are no more under your control than determined ones, and a choice produced by quantum noise is not obviously freer than a choice produced by your character. The problem survives the physics largely intact.