Mr. Justice White
delivered the opinion of the Court.
The Roofing Contractors Association of Southern California, of which respondent was then a member, negotiated a collective bargaining contract with the Roofers Union effective August 15, 1963, establishing compensation levels for the employees of member firms for the next four years. On August 20, 1963, respondent sought to withdraw from the multiple employer bargaining association which had negotiated this agreement. He then refused repeated demands from the union that he sign the contract. At length, the union filed unfair labor practice charges with the National Labor Relations Board, which found that respondent’s refusal to sign the contract which had been negotiated on his behalf by the Association was a violation of §§ 8 (a)(5) and (1) of the National Labor Relations Act, 61 Stat. 140-141, as amended, 29 U. S. C. §§ 158(a)(5) and (1). The Board ordered respondent to sign the contract, cease and desist from unfair labor practices, post notices, and “[p]ay to the appropriate source any fringe benefits provided for in the above-described contract.” 152 N. L. R. B. 9, 14 (1965). The Court of Appeals enforced the Board’s order except as it required the payment of fringe benefits. That part of the order, the Court of Appeals said, “is an order to respondent to carry out provisions of the contract and is beyond the power of the Board.” 386 F. 2d 929, 933 (1967). The Government sought and we granted certiorari as to this holding. 391 U. S. 933 (1968).
Believing the remedy provided by the Board was well within its powers, we reverse the judgment of the Court of Appeals. Section 10 (c) of the Act empowers the Board when it adjudicates an unfair labor practice to issue “an order requiring such person to cease and desist from such unfair labor practice, and to take such affirmative action including reinstatement of employees with or without back pay, as will effectuate the policies of this Act.” 61 Stat. 147, 29 U. S. C. § 160 (c). This grant of remedial power is a broad one. It does not authorize punitive measures, but “[m]aking the workers whole for losses suffered on account of an unfair labor practice is part of the vindication of the public policy which the Board enforces.” Phelps Dodge Corp. v. NLRB, 313 U. S. 177, 197 (1941). Back pay is one of the simpler and more explicitly authorized remedies utilized to attain this end.
Here the unfair labor practice was the failure of the employer to sign and acknowledge the existence of a collective bargaining agreement which had been negotiated and concluded on his behalf. There is no dispute that respondent withdrew from the Roofing Contractors Association too late to escape the binding force of the agreement it had negotiated for him, supplanting previous agreements which had been negotiated in the same way. Nor, in light of the obligation of an employer bargaining in good faith to sign a contract reducing agreed terms to writing, H. J. Heinz Co. v. NLRB, 311 U. S. 514, 524-526 (1941), is it argued that respondent’s failure to sign the agreement was not an unfair labor practice. The judgment of the Board in these respects is not now challenged. The remedy ordered by the Board included a direction to pay the fringe benefits which would have been paid had the employer signed the agreement and thereby recognized his legal obligations which had matured during the collective bargaining process. This is no more than the Act and cases like Phelps Dodge plainly authorize.
The challenge of the employer, in brief, is that ordering the payment of fringe benefits reserved in the contract inserts the Board into the enforcement of the collective bargaining agreement, contrary to the policy and scheme of the statute. Admittedly, the Board has no plenary authority to administer and enforce collective bargaining contracts. Those agreements are normally enforced as agreed upon by the parties, usually through grievance and arbitration procedures, and ultimately by the courts. But the business of the Board, among other things, is to adjudicate and remedy unfair labor practices. Its authority to do so is not “affected by any other means of adjustment or prevention that has been or may be established by. agreement, law, or otherwise . . . .” § 10 (a), 61 Stat. 146, 29 U. S. C. § 160 (a). Hence, it has been made clear that in some circumstances the authority of the Board and the law of the contract are overlapping, concurrent regimes, neither pre-empting the other. NLRB v. C & C Plywood Corp., 385 U. S. 421 (1967); Carey v. Westinghouse Electric Corp., 375 U. S. 261, 268 (1964); Smith v. Evening News Assn., 371 U. S. 195, 197-198 (1962); Teamsters Local 174 v. Lucas Flour Co., 369 U. S. 95, 101, n. 9 (1962). Arbitrators and courts are still the principal sources of contract interpretation, but the Board may proscribe conduct which is an unfair labor practice even though it is also a breach of contract remediable as such by arbitration and in the courts. Smith v. Evening News Assn., 371 U. S. 195, 197-198 (1962). It may also, if necessary to adjudicate an unfair labor practice, interpret and give effect to the terms of a collective bargaining contract. NLRB v. C & C Plywood Corp., 385 U. S. 421 (1967).
Bearing more precisely on this case, the Board is expressly invited by the Act to determine whether an employer has refused to bargain in good faith and thereby violated § 8 (a) (5) by resisting “the execution of a written contract incorporating any agreement reached if requested by either party . . . .” § 8 (d), 61 Stat. 142, 29 U. S. C. § 158 (d); H. J. Heinz Co. v. NLRB, 311 U. S. 514, 524-526 (1941). The Board is not trespassing on forbidden territory when it inquires whether negotiations have produced a bargain which the employer has refused to sign and honor, particularly when the employer has refused to recognize the very existence of the contract providing for the arbitration on which he now insists. To this extent the collective contract is the Board’s affair, and an effective remedy for refusal to sign is its proper business.
Firing an employee for union membership may be a breach of contract open to arbitration, but whether it is or not, it is also an unfair labor practice which may be remedied by reinstatement with back pay under § 10 (c) even though the Board's order mandates the very compensation reserved by the contract. Cf. NLRB v. Great Dane Trailers, Inc., 388 U. S. 26 (1967); Mastro Plastics Corp. v. NLRB, 350 U. S. 270 (1956); Wallace Corp. v. NLRB, 323 U. S. 248 (1944).
The case before us is little, if any, different. The act of refusing to sign the collective bargaining agreement may not have been a breach of contract, but it was an unfair practice. Once adjudicated, it could be remedied by a Board order requiring payment of those fringe benefits which would have been paid had the employer signed and acknowledged the contract which had been duly negotiated on his behalf. The judgment of the Court of Appeals is reversed.
It is so ordered.
Roofers Local 36, United Slate, Tile and Composition Roofers, Damp and Waterproof Workers Association.
See generally Nathanson v. NLRB, 344 U. S. 25, 29-30 (1952); Note, A Survey of Labor Remedies, 54 Va. L. Rev. 38, 41-95 (1968).
Respondent is a past president of the Association, and thus was familiar with its bylaw that a “labor contract negotiated by the Committee shall be binding upon the Regular Members of this Association separately and collectively ...”
The fact that the payments in question here did not constitute direct pay to the employees is irrelevant in our view of this case. Whether the payments were made to the employees, who then contributed them to union trust funds in the form of higher union dues, or whether as here they passed straight from the employer to the trust funds, the final result is the same. And it is just as much in the interest of “effectuat[ing] the policies of this Act,” and of making the employees whole, to require the payments in either case.
Steelworkers Trilogy, 363 U. S. 564, 574, 593 (1960). Congress established the judicial remedy of § 301 of the Labor Management Relations Act, 1947, 61 Stat. 156, 29 U. S. C. § 185, in lieu of a proposal to make breach of a collective bargaining agreement itself an unfair labor practice. H. R. Conf. Rep. No. 510, 80th Cong., 1st Sess., 41-42. The House Conference Report asserts that “[o]nce parties have made a collective bargaining contract the enforcement of that contract should be left to the usual processes of the law and not to the National Labor Relations Board,” id., at 42. See Textile Workers v. Lincoln Mills, 353 U. S. 448, 452 (1957). Cf. LMRA § 201, 61 Stat. 152, 29 TJ. S. C. § 171.