More executive shake-up news today, as Twitter chief Evan Williams hands over the CEO title to Dick Costolo, the company’s existing COO. Williams isn’t leaving Twitter for yet another project–he just wants to focus on Twitter’s products.

It’s an interesting but fitting move for Williams, as Twitter has reached a point of financial promise and aesthetic pleasure. The announcement shouldn’t come as too much of a surprise, given Williams’ comment on how shocked he was at the many pitfalls of Twitter’s website and design.

Focusing on the product itself will ensure Twitter is sticking to its core competencies, and hopefully will avoid getting too caught up in some of the current monetization trends. Twitter has already pulled back from certain initiatives, including @earlybird, indicating the company isn’t afraid to try, fail and improve.

Evan Williams’ full statement is below:

By all accounts Twitter is on a roll. We’ve redesigned our web site to great user feedback. Our user and usage numbers are growing at a rapid clip all around the world. We’ve launched an early, but successful, monetization effort. And, many top engineers, product designers, sales people and other key folks have joined our quickly growing team.

In fact, there are 300 people working at Twitter today—compared to about 20 when I took the CEO job two years ago. Back then, people were creating about 1.25 million tweets a day—compared to 90 million today. In those same two years, we grew from 3 million registered users to more than 160 million today.

The challenges of growing an organization so quickly are numerous. Growing big is not success, in itself. Success to us means meeting our potential as a profitable company that can retain its culture and user focus while having a positive impact on the world. This is no small task. I frequently reflect on the type of focus that is required from everyone at Twitter to get us there.

This led to a realization as we launched the new Twitter. I am most satisfied while pushing product direction. Building things is my passion, and I’ve never been more excited or optimistic about what we have to build.

This is why I have decided to ask our COO, Dick Costolo, to become Twitter’s CEO. Starting today, I’ll be completely focused on product strategy.

When I insisted on bringing Dick into the COO role a year ago, I got a lot of questions from my board. But I knew Dick would be a strong complement to me, and this has proven to be the case. During his year at Twitter, he has been a critical leader in devising and executing our revenue efforts, while simultaneously and effectively making the trains run on time in the office. Dick can be even more effective at this now because Ali Rowghani, Adam Bain, Mike Abbott, Katie Stanton and Kevin Thau joined our leadership team this year and are having a big impact. Given Dick’s track record as a three-time successful CEO, I’m confident we can make this a smooth transition.

I’m extremely proud of how far Twitter has come in the last two years. And, I couldn’t be more excited about where our amazing team will take it next.