9. CAPITAL STOCK AND SHARE-BASED COMPENSATION

We are authorized by our Certificate of Incorporation to issue 60,000,000 shares of common stock, $0.001 par value and 5,000,000 shares of preferred stock, $0.001 par value. As of December 31, 2015, there were 34,786,966 and 33,607,282 shares of common stock issued and outstanding, respectively, and no shares of preferred stock issued or outstanding. Our Board of Directors is authorized to fix the dividend rights and terms, conversion and voting rights, redemption rights and other privileges and restrictions applicable to our preferred stock.

Share-Based Awards

Our 2008 Omnibus Incentive Compensation Plan (the “Plan”) authorizes the grant of various types of equity-based awards, such as stock awards, restricted stock units, stock appreciation rights and stock options to eligible participants, which include all of our employees and all employees of our 50% or more owned subsidiaries, our non-employee directors and certain consultants. The vesting terms of the awards may be tied to continued employment (or, for our non-employee directors, continued service on the Board of Directors) and/or achievement of certain pre-determined performance goals. We refer to stock awards subject to service-based vesting conditions as “non-vested stock” and restricted stock units subject to service-based and performance-based or market-based vesting conditions as “non-vested stock units.” The Plan is administered by the Compensation Committee of our Board of Directors, which determines, within the provisions of the Plan, those eligible employees to whom, and the times at which, awards shall be granted. The Compensation Committee, in its discretion, may delegate its authority and duties under the Plan to specified officers; however, only the Compensation Committee may approve the terms of awards to our executive officers.

Equity-based awards may be granted for a number of shares not to exceed, in the aggregate, approximately 5.5 million shares of common stock, and we had approximately 1.3 million shares available at December 31, 2015. The price per share for stock options shall be of no less than the greater of (a) 100% of the fair value of a share of common stock on the date the option is granted or (b) the aggregate par value of the shares of our common stock on the date the option is granted. If a stock option is granted to any owner of 10% or more of our total combined voting power of us and our subsidiaries, the price is to be at least 110% of the fair value of a share of our common stock on the date the award is granted. Each equity-based award vests ratably over a 12 month-to-six year period, with the exception of those issued under contractual arrangements that specify otherwise, that may be exercised during a period as determined by our Compensation Committee or as otherwise approved by our Compensation Committee. The contractual terms of stock options exercised shall not exceed ten years from the date such option is granted.

Employee Stock Purchase Plan (“ESPP”)

We have a plan whereby our eligible employees may purchase our common stock at 85% of the market price at the time of purchase. On June 7, 2012, our stockholders ratified an amendment adopted by our Board of Directors to increase the total number of shares of our common stock authorized for the issuance under our ESPP from 2,500,000 shares to 4,500,000 shares, and as of December 31, 2015, there were 1,522,288 shares available for future issuance. The following is a detail of the purchases that were made or pending Board of Director approval under the plan:

ESPP expense included in general and administrative expense in our accompanying consolidated statements of operations was $0.4 million, $0.4 million and $0.5 million for 2015, 2014 and 2013, respectively.

Non-Vested Stock Units – Service-Based and Performance-Based Awards

During 2015, we awarded performance-based awards to certain employees. The target level established by the award, which is based on the Company's 2015 adjusted earnings before interest, taxes and depreciation (“EBITDA”), provided for the recipients to receive 154,732 non-vested stock units if the target was achieved. The target number of shares to be potentially awarded has been reduced by forfeitures as indicated in the table below. Performance-based non-vested stock units compensation expense included in general and administrative expenses in our consolidated statements of operations was $1.3 million for 2015.

The weighted average grant date fair value of performance-based non-vested stock units granted was $39.54 in 2015.

At December 31, 2015, there were $4.6 million in unrecognized compensation costs related to our performance-based non-vested stock units that we expect to be recognized over a weighted average period of 2.2 years.

Non-Vested Stock Units – Service-Based and Market-Based Awards

During 2013, we awarded market-based awards to certain employees. The target level established by the award, which is based on our average December 2015 stock price, provided for the recipients to receive 417,330 non-vested stock units if the target is achieved. If the target objective is surpassed to the point of achieving the projected maximum payout, the recipients would receive 667,728 non-vested stock units. The target number of shares to be potentially awarded has been reduced by forfeitures as indicated in the table below.

For market-based awards, the effect of the market condition is reflected in the fair value of the awards at the date of grant using a Monte-Carlo simulation model. A Monte-Carlo simulation model estimates the fair value of the market-based award based upon the expected term, risk-free interest rate and expected volatility. Compensation expense for market-based awards is recognized over the vesting period regardless of whether the market conditions are expected to be achieved. Market-based non-vested stock units compensation expense included in general and administrative expenses in our accompanying consolidated statements of operations was $0.3 million, $0.5 million and $0.8 million for 2015, 2014 and 2013, respectively. The fair value of the 2013 award was estimated using the following assumptions:

The weighted average grant date fair value of market-based non-vested stock units granted was $10.51 in 2013.

At December 31, 2015, there were $0.2 million in unrecognized compensation costs related to our market-based non-vested stock units that we expect to be recognized over a weighted average period of 0.3 years.