EXHIBIT 10.90



 Exhibit 10.90 
 May 16, 2008 
 Mr. James Reagan 
 Deltek,
Inc. 
 13880 Dulles Corner Lane 
 Herndon, VA 20171 

Dear Jim: 
 Reference is made to (i) the
Shareholders’ Agreement, dated as of April 22, 2005 (as it may be amended from time to time, the “Shareholders’ Agreement”), by and among Deltek, Inc., a Delaware corporation (formerly Deltek Systems, Inc., a Virginia
corporation) (the “Company”), the shareholders of the Company listed on the signature pages thereto and, with respect to Sections 3.3 and 3.4 thereof, New Mountain Partners II, L.P., New Mountain Affiliated Investors II, L.P. and
Allegheny New Mountain Partners, L.P. (collectively, the “NMP Entities”) and (ii) the Joinder Agreement dated as of December 15, 2005 (the “Joinder Agreement”) by and among you, the Company and the NMP
Entities, pursuant to which you became a party to the Shareholders’ Agreement. 
 This letter is to confirm our agreement as follows:

 1. Effective November 16, 2008, and conditioned upon your (i) entering into the separation agreement and release letter (the
“Separation and Release”) attached as Attachment A and the Waiver and Release of Claims (the “Waiver and Release”) attached as Attachment B and (ii) re-executing the Waiver and Release on your last day of employment with the
Company, as provided in the Separation and Release, and not revoking the Separation and Release or the Waiver and Release, the Company and the NMP Entities each hereby waives any further compliance by you with, and agrees that you shall have no
further obligations to any of them under, Sections 3.1, 3.3, 3.4, 3.5 and 3.6 of the Shareholders’ Agreement from and after November 16, 2008. 
 2. In consideration of the foregoing, you hereby waive any further compliance by the Company and the NMP Entities with, and agree that the Company and the NMP Entities shall have no further obligations to you under,
Sections 3.1, 3.3, 3.4, 3.5 and 3.6 of the Shareholders’ Agreement from and after November 16, 2008. 
 3. In all other respects,
the Shareholders’ Agreement and the Joinder Agreement shall remain in full force and effect in accordance with their respective terms. 
 If the foregoing corresponds to your understanding of our agreement, please sign this letter and the enclosed copy in the space provided and return one copy to us. 








					
	Very truly yours,
	
	DELTEK, INC.
		
	By:	 	 /s/ Kevin T. Parker

	Name:	 	Kevin Parker
	Title:	 	President and Chief Executive Officer
	
	NEW MOUNTAIN PARTNERS II, L.P.
		
	By:	 	New Mountain Investments II, L.L.C., its general partner
			
		 	By:	 	 /s/ Steven B. Klinsky

		 	Name:	 	Steven B. Klinsky
		 	Title:	 	Managing Member
	
	NEW MOUNTAIN AFFILIATED INVESTORS II, L.P.
		
	By:	 	New Mountain Investments II, L.L.C., its general partner
			
		 	By:	 	 /s/ Steven B. Klinsky

		 	Name:	 	Steven B. Klinsky
		 	Title:	 	Managing Member
	
	ALLEGHENY NEW MOUNTAIN PARTNERS, L.P.
		
	By:	 	New Mountain Investments II, L.L.C., its general partner
			
		 	By:	 	 /s/ Steven B. Klinsky

		 	Name:	 	Steven B. Klinsky
		 	Title:	 	Managing Member


 ACKNOWLEDGED AND AGREED TO 
 AS OF THE DATE FIRST WRITTEN ABOVE 
  

	
	 /s/ James Reagan

	James Reagan