As Amazon celebrates its 20th anniversary,...

...let's take a look at four major challenges it faces on its road ahead.

One: slow international growth particularly in China,...

...where it's failed to make meaningful headway against Chinese e-commerce giant Alibaba...

...and other local players.

Two: new challengers in the US,...

...with online marketplace Jet.com offering a variety of products at a discount to members,...

...and Instacart, a direct competitor to Amazon's Prime Fresh service.

Three: the lack of a desirable mobile phone in its portfolio,...

...which leaves it completely dependent on others as more shopping moves to mobile.

And four: the strong chance that brick-and-mortar retailers like Walmart...

...finally develop cohesive and successful e-commerce strategies.

Amazon has been enjoying double-digit revenue growth,...

...but that growth has slowed in recent years.

Its 2010 to 2011 growth of 41% went down to more than half...

...to only 20% growth from 2013 to 2014,...

...suggesting that some of these headwinds may be impacting the company.