For almost any period of market history, whether dating back to 1924 or the beginning of 2017, just a handful of companies have accounted for a sizable amount of all stock market gains. Individual investors may have realized that their own portfolios have echoed that trend as well, serving as a microcosm of the broader market. Chances are your biggest winner has made up for a handful of losers and mediocre stocks.

That's one reason why growth stocks are so important to a portfolio. If you buy a great business early enough its long-term trajectory, then you could see compounding gains for years to come. While that's easier said than done, I think there are promising growth stocks to buy in any market -- including the relatively expensive one heading into 2018. Here's why investors looking for growth should take a closer look at productivity software leader Atlassian (NASDAQ:TEAM), solar hardware manufacturer SolarEdge (NASDAQ:SEDG), and biopharma supplier Repligen (NASDAQ:RGEN).

An overhead view of two people working together at a desk.

Image source: Getty Images.

Buying opportunities emerging

Investors have enjoyed fireworks with nearly every quarterly update from Atlassian, which continues to grow at an unbelievable clip. Fiscal first-quarter 2018 revenue for the period ended September 2017 soared 42% from the year-ago period, putting the company on pace for over $800 million of revenue for the full year. Considering total revenue totaled $619 million last year and $457 million the year before, the software provider is quietly one of the fastest-growing companies of its size.  

A steady stream of acquisitions, new product launches, and (importantly) a focus on building the community aspect of its user base have combined to fuel the incredible growth to date. But the $10 billion company could find plenty of room to gallop in the years ahead. The business communication and collaboration software market is expected "to grow from $15 billion in 2016 to more than $28 billion in 2020," according to a 451 Research market report discussed by Wired. Atlassian is not only driving the market, but also capturing market share along the way -- an enviable position for any growth stock.

It's worth pointing out that management has taken full advantage of Wall Street's adoration by using one of the oldest tech business models ever crafted: growth over everything -- including profits. Atlassian's carefully managed growth in the years leading up to its IPO have since given way to mounting losses. That hasn't mattered much lately, as cash flow has been incredible and, if push came to shove, the company could cut expenses relatively easily. But if it crops up as an issue with Wall Street at any point in 2018, then it could create additional opportunities for long-term investors to buy shares at a discount.

A worker installing rooftop solar panels.

Image source: Getty Images.

Growth with an edge

SolarEdge has carved out an impressive niche within the solar industry with its portfolio of hardware products. While solar panels and dreams of energy storage products usually capture the imagination of investors in the space, selling inverters, power optimizers, and module-level monitoring services is proving to be a lucrative business for the company. The stock has gained nearly 180% in the last year on impressive top-line growth and eye-popping gross margins.

What's driving the virtually overnight market traction for SolarEdge? Its gadgets and software know-how provide mission-critical functionality for a solar installation, such as converting direct current from a panel to the alternating current used by the American grid, simplifying setups by combining multiple products into one, and optimizing when electricity is distributed back to the utility or the customer. In other words, the company's products serve as the brains of photovoltaic system, which has allowed solar panel manufacturers to focus on, well, solar panels.

The incredible growth potential of solar energy globally is great news for the $1.7 billion company. That's especially true considering 51% of its third-quarter 2017 revenue -- a quarterly record of $166 million -- was sourced from customers outside of the United States. More impressive is that SolarEdge is growing the top line in a healthy and sustainable way: It reported gross margin of 34.9%, net income of $28 million, and operating cash flow of $33.6 million for the most recent quarter. 

Simply put, there's not a whole lot to complain about for shareholders, and there's no sign that the growth will stop anytime soon.

A lab worker pointing at a touchscreen in the plane of the image, a strand of DNA is displayed to the right.

Image source: Getty Images.

Biopharma growth without the clinical risks

Most investors have probably never heard of Repligen, but it has quietly been an incredible growth stock in the last five years, boasting returns of 465%. The company is a leading supplier of bioprocess purification products that are required to manufacture biologic drugs. Positioning itself as a partner the biopharma industry cannot live without has allowed shareholders to enjoy the awesome growth of the sector without the binary risks inherent to clinical trials.

In fact, Repligen's products are currently being used in the biomanufacturing of over 70 biologic drugs on the market and 300 in clinical development. While not all of those drug candidates will reach the market, the ones that do have the potential to capture customers for a decade or more -- or however long it takes for the intellectual property to expire. Even that biopharma risk is mitigated by the business model. After all, purification costs are not related to drug price, so generic competition for one customer down the road is actually a boon for business, as it will simply create more customers for the bioprocess technology leader.

As far as biopharma stocks go, the long-term growth potential for Repligen is amazingly assured. But the near-term growth shouldn't be overlooked, either. That's because a recent acquisition is expected to boost 2018 product revenue by up to $50 million at gross margin exceeding 55%. Considering product revenue for the first nine months of 2017 reached $99.5 million, next year promises to be another record year for the business. And since the stock price has dropped in the second half of the year, investors looking for growth could be staring at a solid buying opportunity. 

Investor takeaway

You don't need the power of hindsight to find promising growth stocks that can drive the bulk of your portfolio's long-term gains. It's not always easy to find great businesses with growth potential that's relatively certain, but it can be done with the proper framework. For instance, all three stocks on this list have healthy levels of cash flow to support their continued growth, which provides a lot of financial flexibility to invest in expansion or respond to souring market conditions.

Two of the companies -- SolarEdge and Repligen -- are easily profitable thanks to their high-margin businesses driven by niche positions within their respective industries. They prove you can reap the rewards of high-growth markets (solar power and biopharma) by investing in the lesser known elements that provide the backbone of the industry. Meanwhile, Atlassian is the only stock of the bunch reporting losses, but that's because it's in the enviable position of simultaneously growing an entire industry and capturing market share.

If you're looking for growth stocks to buy in 2018, you should strongly consider these three companies.

Maxx Chatsko has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Atlassian. The Motley Fool has a disclosure policy.