Condensed Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 29, 2015	Mar. 30, 2014	Mar. 29, 2015	Mar. 30, 2014
Condensed Consolidated Statements of Income
Revenue	$ 564,763	$ 586,285	$ 1,145,819	$ 1,232,133
Subcontractor costs	(132,009)	(130,300)	(275,985)	(293,158)
Other costs of revenue	(362,957)	(386,913)	(721,238)	(783,442)
Selling, general and administrative expenses	(42,512)	(44,229)	(84,699)	(91,602)
Contingent consideration - fair value adjustments	3,113	21,343	3,113	25,973
Operating income	30,398	46,186	67,010	89,904
Interest expense	(1,804)	(2,496)	(3,594)	(4,919)
Income before income tax expense	28,594	43,690	63,416	84,985
Income tax expense	(9,584)	(11,781)	(18,760)	(25,749)
Net income including noncontrolling interests	19,010	31,909	44,656	59,236
Net loss (income) attributable to noncontrolling interests	7	(200)	(64)	(213)
Net income attributable to Tetra Tech	$ 19,017	$ 31,709	$ 44,592	$ 59,023
Earnings per share attributable to Tetra Tech:
Basic	$ 0.31	$ 0.49	$ 0.72	$ 0.91
Diluted	$ 0.31	$ 0.48	$ 0.71	$ 0.90
Weighted-average common shares outstanding:
Basic	61,153	64,835	61,816	64,670
Diluted	61,723	65,710	62,431	65,517
Cash dividends paid per share	$ 0.07		$ 0.14