9. Stock-Based Compensation

The Company provides equity-based compensation awards to employees, independent contractors and directors as an effective means for attracting, retaining and motivating such individuals. The Company maintains two share-based compensation plans: the 2014 Plan and the 2008 Stock Incentive Plan (the “2008 Plan”). Upon the effective date of the 2014 Plan in January 2014, the Company ceased using the 2008 Plan to grant new equity awards and began using the 2014 Plan for grants of new equity awards.

Employee Stock Purchase Plan

The Company’s 2017 Employee Stock Purchase Plan (the “ESPP”) provides (i) for two offering periods each year and (ii) that the purchase price for shares of the Company’s common stock purchased under the ESPP will be 90% of the lesser of the fair market value of 2U’s common stock on the purchase date or the fair market value of 2U’s common stock on the first day of the offering period. Participating eligible employees select a rate of payroll deduction between 1% and 15% of their salary or wage compensation received from the Company as in effect at the start of the offering period, subject to a maximum payroll deduction per calendar year of $25,000. Participation in the ESPP began on January 1, 2018.

Stock-Based Compensation Expense

Stock-based compensation expense related to stock-based awards is included in the following line items in the condensed consolidated statements of operations and comprehensive loss:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(in thousands)
Curriculum and teaching	$	5	$	—	$	7	$	—
Servicing and support	1,320		1,161		2,192		1,856
Technology and content development	1,075		897		1,786		1,543
Marketing and sales	718		430		1,207		772
General and administrative	5,891		3,007		10,939		5,219

Total stock-based compensation expense	$	9,009	$	5,495	$	16,131	$	9,390

Stock Options

The following is a summary of the stock option activity for the six months ended June 30, 2018:

	Number of Options		Weighted-Average Exercise Price per Share
Outstanding balance at December 31, 2017	4,559,176		$	15.10
Granted	655,524		84.12
Exercised	(501,531	)	9.56
Forfeited	(53,531	)	35.69
Expired	—		—

Outstanding balance at June 30, 2018	4,659,638		25.17

Exercisable at June 30, 2018*	3,201,819		11.26

*As of June 30, 2018, the aggregate intrinsic value of options exercisable was $231.5 million and such shares had a weighted-average remaining contractual term of 4.73 years.

Restricted Stock Units

The following is a summary of restricted stock unit activity for the six months ended June 30, 2018:

	Number of Restricted Stock Units		Weighted- Average Grant Date Fair Value
Outstanding balance at December 31, 2017	1,413,423		$	29.95
Granted	440,818		81.39
Vested	(517,050	)	23.34
Forfeited	(56,564	)	36.38

Outstanding balance at June 30, 2018	1,280,627		50.04