Earnings Per Share (EPS) (Details) (USD $) In Thousands, except Share data	3 Months Ended	9 Months Ended
Jul. 03, 2011	Jun. 27, 2010	Jul. 03, 2011	Jun. 27, 2010
Earnings Per Share ("EPS")
Net income attributable to Tetra Tech	$ 23,839	$ 20,639	$ 63,639	$ 53,678
Weighted-average common shares outstanding - basic (in shares)	62,203,000	61,560,000	61,967,000	61,380,000
Effect of dilutive stock options and unvested restricted stock (in shares)	731,000	621,000	778,000	735,000
Weighted-average common stock outstanding - diluted (in shares)	62,934,000	62,181,000	62,745,000	62,115,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.38	$ 0.34	$ 1.03	$ 0.87
Diluted (in dollars per share)	$ 0.38	$ 0.33	$ 1.01	$ 0.86
Options excluded from the calculation of dilutive potential common shares (in shares)	2,900,000	2,100,000	2,700,000	2,300,000

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) in the future that were not included in the computation of diluted EPS because to do so would increase EPS amounts or decrease loss per share amounts for the period presented.

+ References

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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 128
-Paragraph 13, 14

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The amount of net income or loss for the period per each share of common stock outstanding during the reporting period.

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The amount of net income or loss for the period per each share of common stock and dilutive common stock equivalents outstanding during the reporting period.

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-Publisher FASB
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Aggregate awards of share options and nonvested shares to be issued to an employee under a share-based compensation arrangement are considered options for purposes of computing diluted earnings per share. Such share-based awards shall be considered to be outstanding as of the grant date for purposes of computing diluted earnings per share even though their exercise may be contingent upon vesting. Those share-based awards are included in the diluted Earnings Per Share (EPS) computation even if the employee may not receive (or be able to sell) the stock until some future date. Accordingly, all shares to be issued shall be included in computing diluted EPS if the effect is dilutive. The dilutive effect of share-based compensation arrangements shall be computed using the treasury stock method. If the equity share options or other equity instruments are outstanding for only part of a period, the shares issuable shall be weighted to reflect the portion of the period during which the equity instruments were outstanding.

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The portion of consolidated profit or loss for the period, net of income taxes, which is attributable to the parent. If the entity does not present consolidated financial statements, the amount of profit or loss for the period, net of income taxes.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The average number of shares issued and outstanding that are used in calculating diluted EPS, determined based on the timing of issuance of shares in the period.

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Number of [basic] shares, after adjustment for contingently issuable shares and other shares not deemed outstanding, determined by relating the portion of time within a reporting period that common shares have been outstanding to the total time in that period.

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