Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

BARON INVESTMENT FUNDS TRUST (f/k/a BARON ASSET FUND)

Prospectus Date

rr_ProspectusDate

Jan. 28, 2015

Supplement [Text Block]

bift_SupplementTextBlock

BARON

FUNDS^®

Baron Opportunity Fund

Supplement dated April 30, 2015

to Prospectus dated January 28, 2015

Effective April 30, 2015, the Prospectus is modified as follows:

On page 25 of the Prospectus, the “Average Annual Total Returns for the periods ended December 31, 2014” table and the paragraph that follows the table are deleted in their entirety and replaced with the following:

	1 year			5 years			10 years			Since Inception
BARON OPPORTUNITY FUND
Retail Shares (Inception date: 02-29-00)
Return before taxes		(1.67	)%		12.60	%		8.67	%		5.10	%
Return after taxes on distributions		(2.95	)%		11.80	%		8.28	%		4.84	%
Return after taxes on distributions and sale of Fund shares		0.07	%		9.95	%		7.04	%		4.11	%
Institutional Shares* (Inception date: 05-29-09)
Return before taxes		(1.40	)%		12.90	%		8.83	%		5.21	%
Russell 3000® Growth Index (reflects no deduction for fees, expenses or taxes)		12.44	%		15.89	%		8.50	%		2.29	%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		13.69	%		15.45	%		7.67	%		4.79	%
Russell Midcap® Growth Index** (reflects no deduction for fees, expenses or taxes)		11.90	%		16.94	%		9.43	%		3.70	%

* Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

** Baron Opportunity Fund no longer considers the Russell Midcap^® Growth Index an appropriate benchmark index. The Russell Midcap^® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000^® Growth Index is more representative of the Fund’s current investable universe.

The Russell 3000^® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies. The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies.

This information supplements the Prospectus dated January 28, 2015. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.

BARON OPPORTUNITY FUND

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

bift_SupplementTextBlock

BARON

FUNDS^®

Baron Opportunity Fund

Supplement dated April 30, 2015

to Prospectus dated January 28, 2015

Effective April 30, 2015, the Prospectus is modified as follows:

On page 25 of the Prospectus, the “Average Annual Total Returns for the periods ended December 31, 2014” table and the paragraph that follows the table are deleted in their entirety and replaced with the following:

	1 year			5 years			10 years			Since Inception
BARON OPPORTUNITY FUND
Retail Shares (Inception date: 02-29-00)
Return before taxes		(1.67	)%		12.60	%		8.67	%		5.10	%
Return after taxes on distributions		(2.95	)%		11.80	%		8.28	%		4.84	%
Return after taxes on distributions and sale of Fund shares		0.07	%		9.95	%		7.04	%		4.11	%
Institutional Shares* (Inception date: 05-29-09)
Return before taxes		(1.40	)%		12.90	%		8.83	%		5.21	%
Russell 3000® Growth Index (reflects no deduction for fees, expenses or taxes)		12.44	%		15.89	%		8.50	%		2.29	%
S&P 500 Index (reflects no deduction for fees, expenses or taxes)		13.69	%		15.45	%		7.67	%		4.79	%
Russell Midcap® Growth Index** (reflects no deduction for fees, expenses or taxes)		11.90	%		16.94	%		9.43	%		3.70	%

* Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

** Baron Opportunity Fund no longer considers the Russell Midcap^® Growth Index an appropriate benchmark index. The Russell Midcap^® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000^® Growth Index is more representative of the Fund’s current investable universe.

The Russell 3000^® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies. The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies.

This information supplements the Prospectus dated January 28, 2015. This Supplement and the Prospectus constitute a current prospectus. To request another copy of the Prospectus, please call 1-800-992-2766 or visit our website at www.BaronFunds.com.

Performance Table Market Index Changed

rr_PerformanceTableMarketIndexChanged

Baron Opportunity Fund no longer considers the Russell Midcap^® Growth Index an appropriate benchmark index. The Russell Midcap^® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000^® Growth Index is more representative of the Fund’s current investable universe.

Performance Table Closing [Text Block]

rr_PerformanceTableClosingTextBlock

The Russell 3000^® Growth Index is an unmanaged index of companies classified as growth among the largest 3,000 U.S. companies. The S&P 500 Index is an unmanaged index of larger-cap companies. The Russell Midcap^® Growth Index is an unmanaged index of mid-cap growth companies.

BARON OPPORTUNITY FUND | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(1.67%)

5 years

rr_AverageAnnualReturnYear05

12.60%

10 years

rr_AverageAnnualReturnYear10

8.67%

Since Inception

rr_AverageAnnualReturnSinceInception

5.10%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 29, 2000

BARON OPPORTUNITY FUND | Institutional Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(1.40%)

^[1]

5 years

rr_AverageAnnualReturnYear05

12.90%

^[1]

10 years

rr_AverageAnnualReturnYear10

8.83%

^[1]

Since Inception

rr_AverageAnnualReturnSinceInception

5.21%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2009

^[1]

BARON OPPORTUNITY FUND | Return after taxes on distributions | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

(2.95%)

5 years

rr_AverageAnnualReturnYear05

11.80%

10 years

rr_AverageAnnualReturnYear10

8.28%

Since Inception

rr_AverageAnnualReturnSinceInception

4.84%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 29, 2000

BARON OPPORTUNITY FUND | Return after taxes on distributions and sale of Fund shares | Retail Shares

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

0.07%

5 years

rr_AverageAnnualReturnYear05

9.95%

10 years

rr_AverageAnnualReturnYear10

7.04%

Since Inception

rr_AverageAnnualReturnSinceInception

4.11%

Inception Date

rr_AverageAnnualReturnInceptionDate

Feb. 29, 2000

BARON OPPORTUNITY FUND | Russell 3000® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

12.44%

5 years

rr_AverageAnnualReturnYear05

15.89%

10 years

rr_AverageAnnualReturnYear10

8.50%

Since Inception

rr_AverageAnnualReturnSinceInception

2.29%

BARON OPPORTUNITY FUND | S&P 500 Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

13.69%

5 years

rr_AverageAnnualReturnYear05

15.45%

10 years

rr_AverageAnnualReturnYear10

7.67%

Since Inception

rr_AverageAnnualReturnSinceInception

4.79%

BARON OPPORTUNITY FUND | Russell Midcap® Growth Index (reflects no deduction for fees, expenses or taxes)

Risk/Return:

rr_RiskReturnAbstract

1 year

rr_AverageAnnualReturnYear01

11.90%

^[2]

5 years

rr_AverageAnnualReturnYear05

16.94%

^[2]

10 years

rr_AverageAnnualReturnYear10

9.43%

^[2]

Since Inception

rr_AverageAnnualReturnSinceInception

3.70%

^[2]

[1] Performance for the Institutional Shares prior to May 29, 2009 is based on the performance of the Retail Shares, which have a distribution fee. The Institutional Shares do not have a distribution fee. If the annual returns for the Institutional Shares prior to May 29, 2009 did not reflect this fee, the returns would be higher.

[2] Baron Opportunity Fund no longer considers the Russell Midcap® Growth Index an appropriate benchmark index. The Russell Midcap® Growth Index is included in the table above for comparison purposes for the period before Baron Opportunity Fund converted to an all-cap fund. Prior to February 20, 2015, the Fund invested in companies with market capitalizations between $1 billion and $15 billion at the time of purchase. Since then, the Fund may invest in companies of all market capitalizations. The Adviser believes that the Russell 3000® Growth Index is more representative of the Fund’s current investable universe.