Personal services are not delegable. If the contract is such that the promisee expects the obligor personally to perform the duty, the obligor may not delegate it. Suppose the Catskill Civic Opera Association hires a famous singer to sing in its production of Carmen and the singer delegates the job to her understudy. The delegation is ineffective, and performance by the understudy does not absolve the famous singer of liability for breach.
Many duties may be delegated, however. Indeed, if they could not be delegated, much of the world’s work would not get done. If you hire a construction company and an architect to design and build your house to certain specifications, the contractor may in turn hire individual craftspeople—plumbers, electricians, and the like—to do these specialized jobs, and as long as they are performed to specification, the contract terms will have been met. If you hired an architecture firm, though, you might not be contracting for the specific services of a particular individual in that firm.