Condensed Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Apr. 01, 2012	Mar. 31, 2013	Apr. 01, 2012
Condensed Consolidated Statements of Income
Revenue	$ 641,999	$ 624,345	$ 1,300,544	$ 1,306,972
Subcontractor costs	(121,052)	(147,453)	(282,399)	(338,024)
Other costs of revenue	(435,827)	(388,915)	(844,822)	(796,251)
Selling, general and administrative expenses	(47,453)	(52,434)	(93,847)	(101,062)
Operating income	37,667	35,543	79,476	71,635
Interest expense - net	(2,136)	(1,453)	(3,320)	(2,763)
Income before income tax expense	35,531	34,090	76,156	68,872
Income tax expense	(10,659)	(11,769)	(24,888)	(23,848)
Net income including noncontrolling interests	24,872	22,321	51,268	45,024
Net income attributable to noncontrolling interests	(52)	(37)	(225)	(130)
Net income attributable to Tetra Tech	$ 24,820	$ 22,284	$ 51,043	$ 44,894
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.38	$ 0.35	$ 0.79	$ 0.71
Diluted (in dollars per share)	$ 0.38	$ 0.35	$ 0.78	$ 0.71
Weighted-average common shares outstanding:
Basic (in shares)	64,551	63,072	64,376	62,846
Diluted (in shares)	65,472	63,817	65,208	63,497