D operates a large maintenance and overhaul base in Kansas City. In 1967, P was hired by D to work as a clerk in the Stores Department at its Kansas City base. The Stores Department must operate 24 hours per day, 365 days per year, and whenever an employee's job in that department is not filled, an employee must be shifted from another department, or a supervisor must cover the job, even if the work in other areas may suffer. P was subject to a seniority system contained in a collective-bargaining agreement. Employees bid for particular shift assignments with the most senior employees having first choice for job and shift assignments, and the most junior employees are required to work when the union steward is unable to find enough people willing to work at a particular time or in a particular job to fill D's needs. P was a member of a religion known as the Worldwide Church of God (Jehovah’s Witness). Worldwide observes the Sabbath by refraining from performing any work from sunset on Friday until sunset on Saturday. P informed and the problem was temporarily solved when P transferred to the 11 p.m.-7 a.m. shift. Working this shift permitted P to observe his Sabbath. But P bid for and received a transfer from Building 1, where he had been employed, to Building 2, where he would work the day shift. The two buildings had entirely separate seniority lists. In Building 1 P had sufficient seniority to observe the Sabbath regularly, he was second from the bottom on the Building 2 seniority list. P was asked to work Saturdays. But the union was not willing to violate the seniority provisions set out in the collective-bargaining contract, and P had insufficient seniority to bid for a shift having Saturdays off. P proposed a four days a week work schedule but D rejected that because P's job was essential, and on weekends he was the only available person on his shift to perform it. To fill P's position with a supervisor or an employee from another area would simply have undermanned another operation, and to employ someone not regularly assigned to work Saturdays would have required D to pay premium wages. P refused to report for work on Saturdays. After a hearing, P was discharged on grounds of insubordination for refusing to work during his designated shift. P's claim of religious discrimination rested on 1967 EEOC guidelines requiring employers 'to make reasonable accommodations to the religious needs of employees' whenever such accommodation would not work an 'undue hardship,' and on similar language adopted by Congress in the 1972 amendments to Title VII, 42 U.S.C. § 2000e(j) (1970 ed., Supp. V). The District Court ruled in favor of D. It ruled that D had satisfied its 'reasonable accommodations' obligation, and any further accommodation would have worked an undue hardship on the company. The Court of Appeals reversed the judgment holding that D had not satisfied its duty to accommodate. The Supreme Court granted certiorari.