In JavaScript, both `prototype` and `__proto__` (often accessed using the `Object.getPrototypeOf()` and `Object.setPrototypeOf()` methods) are related to the prototype chain and object inheritance, but they have different roles and purposes. 1. `prototype`: The `prototype` property is a property of a constructor function, such as `function Person() {}`, and is used to define and store properties and methods that will be inherited by objects created using that constructor function. It is an object that serves as a blueprint for creating other objects. When you create an instance of a constructor function using the `new` keyword, the newly created object's `__proto__` property is set to the `prototype` property of the constructor function. Example: 

function Person(name) {
  this.name = name;
}

Person.prototype.greet = function() {
  console.log(`Hello, my name is ${this.name}.`);
};

const person = new Person('Alice');
person.greet();  // Hello, my name is Alice.
In this example, `Person.prototype` is the prototype object associated with the `Person` constructor function. The `greet()` method defined on `Person.prototype` is then inherited by objects created with `new Person()`. 2. `__proto__`: The `__proto__` property is a property of individual objects that references the prototype of the object. It is a reference to the object's internal [[Prototype]] property. It allows objects to access properties and methods defined on their prototype. Example: 

const person = { name: 'Alice' };
console.log(person.__proto__ === Object.prototype);  // true

person.toString();  // inherited from Object.prototype
In this example, `Object.prototype` is the prototype object for all objects created in JavaScript, including `person`. The `__proto__` property of `person` references `Object.prototype`, allowing `person` to access methods like `toString()` defined on `Object.prototype`. It's important to note that while `__proto__` is a non-standard property, it is widely supported in browsers. It has been standardized as `[[Prototype]]` in the ECMAScript specification, and you should prefer using the `Object.getPrototypeOf()` and `Object.setPrototypeOf()` methods to access and set the prototype of an object. In summary, `prototype` is a property of a constructor function used to define properties and methods that will be inherited by instances created with that constructor, while `__proto__` is a property of individual objects that references their prototype in the prototype chain.