Small-cap stocks can offer investors impressive gains, but they also come with their fair share of risk. That makes it particularly important to research small-cap companies before putting them in your portfolio. However, knowing which small caps to consider can be challenging, so we asked some of our top Motley Fool contributors which companies will be on their radars in 2015. Read on for the details.

Leo Sun
Geron (NASDAQ:GERN) is a key small-cap biotech to watch in 2015. It initially developed stem-cell-based treatments, but it halted those operations in 2011 and sold the entire pipeline to BioTime (NYSEMKT:BTX) subsidiary Asterias Biotherapeutics in 2013. Geron then focused on developing two cancer drugs: GRN1005 and imetelstat. However, Geron eventually scrapped GRN1005 and halted imetelstat's trials for breast and lung cancer.

But Geron didn't give up. It continued imetelstat's phase 2 trials for various blood disorders including myelofibrosis, a bone marrow disorder which disrupts the production of red blood cells. Unfortunately, the FDA placed a full clinical hold on imetelstat early in 2014, causing many investors to abandon the stock.

Yet the company bounced back in November thanks to three huge catalysts: The hold was lifted, Geron reported that seven patients (21.2% of all patients in the trial) achieved a complete or partial response, and it signed a licensing deal with Johnson & Johnson's (NYSE:JNJ) Janssen unit. The J&J deal entitles Geron -- which only reported $1.3 million in revenue (licenses and royalties) last year -- to $935 million in potential milestone payments and "low double-digit" royalties for imetelstat if it is commercialized.

Geron isn't a safe stock by any means. It's still a one-drug play that could easily crash on any negative news -- but it's definitely one biotech investors should keep an eye on.

Dan Caplinger
One small-cap stock I recently added to my portfolio is Enphase Energy (NASDAQ:ENPH), a player in the solar industry that stands to gain a lot from the surge in interest in residential and small-commercial solar power systems. Currently, the direct-current electricity generated by solar power systems must be converted into alternating-current form for use. In the past, systems relied on large central inverters that converted the power from multiple solar panels. But Enphase helped pioneer microinverter technology, which allows solar systems to convert power generated at the individual solar-panel level. That results in greater power generation than central inverters, making solar power systems more attractive.

Enphase holds a domestic market share of more than 90% of residential systems that use microinverters. Yet Enphase still has substantial growth prospects, because only about half of residential systems today use microinverter technology. In addition, Enphase has not tapped the market for commercial systems to nearly the same extent it has achieved success in the residential arena. Competition remains fierce, and the rapid evolution of technology in the solar industry will require Enphase to keep up with big players in the space. Nevertheless, coming from a position of strength, Enphase has a lot of potential.

Todd Campbell
There's no doubt in my mind that many of today's small-cap stocks will get much bigger over the next year. While Leo and Dan made compelling cases for their two picks, I'm betting on Ophthotech (NASDAQ:OPHT), which is working on a promising drug for vision loss.

Ophthotech's lead drug candidate is Fovista, which is being studied as a treatment for age-related macular degeneration, or AMD. That is a big indication. Novartis' Lucentis racks up about $4 billion a year in sales treating AMD and other vision loss conditions, including diabetic macular edema. Regeneron's Eylea, which also treats both AMD and DME, is poised to generate $2.8 billion in sales in the coming year, too. What's intriguing about Ophthotech, though, is that Fovista is being studied as a treatment to be used alongside those drugs, rather than instead of them. In midstage studies, Fovista dosed alongside Lucentis did a better job at improving vision than Lucentis alone. If Ophthotech can deliver similar results in phase 3, Fovista could be in a great position to become a top seller. Novartis appears to agree: It just inked a deal that could be worth as much as $1 billion to Ophthotech, plus royalties, just to secure Fovista's international rights. Novartis' vote of confidence is just one more reason to watch Ophthotech this year.