Condensed Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Apr. 01, 2012	Apr. 03, 2011	Apr. 01, 2012	Apr. 03, 2011
Revenue	$ 624,345	$ 612,566	$ 1,306,972	$ 1,223,690
Subcontractor costs	(147,453)	(182,260)	(338,024)	(387,805)
Other costs of revenue	(388,915)	(355,445)	(796,251)	(685,372)
Selling, general and administrative expenses	(52,434)	(45,605)	(101,062)	(86,932)
Operating income	35,543	29,256	71,635	63,581
Interest expense - net	(1,453)	(1,471)	(2,763)	(2,776)
Income before income tax expense	34,090	27,785	68,872	60,805
Income tax expense	(11,769)	(9,704)	(23,848)	(19,970)
Net income including noncontrolling interests	22,321	18,081	45,024	40,835
Net income attributable to noncontrolling interests	(37)	(581)	(130)	(1,034)
Net income attributable to Tetra Tech	$ 22,284	$ 17,500	$ 44,894	$ 39,801
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.35	$ 0.28	$ 0.71	$ 0.64
Diluted (in dollars per share)	$ 0.35	$ 0.28	$ 0.71	$ 0.64
Weighted-average common shares outstanding:
Basic (in shares)	63,072	62,121	62,846	61,836
Diluted (in shares)	63,817	62,945	63,497	62,638