Exhibit 10.21
April 22, 2013

George Stromeyer

Dear George,

I am pleased to offer you the position of Senior Vice President, Worldwide
Sales, reporting to our President and Chief Executive Officer, Patrick Harshman.
You will receive a starting bi-weekly salary of $10,576.92, which equates to
$275,000.00 on an annualized basis (for computational purposes only). In
addition, you will have an annual cash incentive target of 100% of your annual
salary, which will be dependent upon Harmonic’s achievement of certain corporate
financial objectives and your direct contributions toward achieving these
objectives as will be set forth in your incentive plan. For the 2013 incentive
plan year, your incentive will be paid on a pro-rata basis calculated from your
date of hire, except that your incentive will be guaranteed from the period
between your employment commencement date and September 30, 2013. Except as
provided in this letter, any earned incentive will be paid in accordance with
the terms and conditions of your incentive plan. All of the above amounts are
subject to federal and state tax withholdings.

Upon commencement of employment, you will be eligible for stock options
entitling you to purchase 100,000 shares of Harmonic Common Stock. The exercise
price will be the closing market price of Harmonic’s shares on the date of
approval. The options will vest incrementally over a three (3)- year period
subject to your continued service with Harmonic, with 1/3 vesting upon the first
anniversary of your employment commencement date and the remaining balance of
the options vesting over the following (2)-year period with vesting occurring in
equal monthly installments. Also, upon commencement of employment, you will be
eligible to receive 60,000 restricted stock units (RSUs), each unit representing
one share of Harmonic Common Stock. These RSUs will vest over a two (2)-year
period subject to your continued service with Harmonic, with 50% vesting upon
the first anniversary of your employment commencement date and the remaining 50%
of the RSUs vesting on the second anniversary of your employment commencement
date. These option and RSU grants will be subject to the terms and conditions of
Harmonic’s 1995 Stock Plan, as amended, and the form of award agreements
approved for making option and RSU grants thereunder. The option and RSU grants
described above are subject to approval by the Compensation and Equity Ownership
Committee of the Board of Directors of Harmonic.

Additionally, within 15 days of your employment commencement date, you will
receive a one- time lump sum signing bonus of $75,000.00 (less applicable
federal and state tax withholdings). You agree and acknowledge that in the event
of your resignation prior to your first year anniversary of employment or in the
event that your employment is terminated for “Cause” prior to your first
anniversary of employment, you will repay this signing bonus to Harmonic on a
pro-rata basis as calculated from your employment start date to the termination
date.

Moreover, you will be offered to become a party to Harmonic’s standard executive
Change of Control agreement. Details of this agreement will be communicated to
you under separate cover.

Harmonic offers a comprehensive benefits package including health and welfare
plans along with our 401(k) thrift savings plan. We reserve the right to modify
or change our benefits at any time in our sole discretion. We will provide you
with additional information regarding our complete list of our current fringe
benefits during your orientation session.

If you decide to accept this offer of employment with the Company, you are
required to sign and comply with the Company’s At-Will Employment, Confidential
Information, Invention Assignment, and Arbitration Agreement (“Confidentiality
Agreement”). A copy of the Confidentiality Agreement has been attached as
Exhibit A.

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Before starting, you must sign the Confidentiality Agreement and bring
documentation for completion of the I-9 (employment verification) form. Your
employment with Harmonic is at will, which means either party can choose to
terminate the relationship at any time for any reason whatsoever, with or
without cause.

This offer of employment is valid until the close of business on May 3, 2013,
and is contingent upon the satisfactory completion of reference checks and a
background check.

George, we believe you will play a key role in the growth and success of
Harmonic going forward; we are very much looking forward to working with you.
Please let me know of your acceptance by signing a copy of this offer letter and
returning to me via scan or confidential direct fax to 408.490.6352.

Sincerely,

/s/ Peter E. Hilliard

Peter E. Hilliard
Senior Vice President, Worldwide Human Resources
Harmonic Inc.

Accepted and Anticipated Start Date:

/s/ George Stromeyer April 24, 2013
George Stromeyer    Date

Anticipated Start Date: June 3, 2013