A startup has the most uncertainty in the beginning. You have no customers. Your team may be incomplete. You may not even fully understand what your market is or even your product. If you’re an entrepreneur, you’re either in this situation now, or you’ve been there before.

Jessica Livingston’s book, “Founder’s at Work” profiles some of the most famous startups of the past few decades (and some you didn’t know) by interviewing founders and asking them what the “early days” were like.

Despite hearing stories from 32 different companies, ranging from hardware to software to consumer web, there were a few common themes I noticed throughout:

1) You’re idea will change.

Whether it was PayPal, Apple, or 37 Signals, where you start out may not be where you finish.  Paypal originally thought they’d manage transactions on the Palm Pilot. 37 Signals was original a web development consulting firm. Apple thought they’d sell some kits to computer hobbyists.  We all know how all those turned out. Don’t be surprised if your startup experiences the same thing.

2) Not every great business knows how they’ll make money from the start.

Blogger, Trip Advisor (an exhibitor at the Career Combine!) and many others struggled greatly with finding the right revenue model.  Does that mean we should all adopt the Underpants Gnomes business model? No. But it does mean that you should be open to adapting and trying multiple revenue opportunities.  Trip Advisor even went as far as walking away from some deals when they decided a different path was the best model, even though it was unproven and would take longer to bring in revenue.

3) Choosing the right investor is important.

There are a few horror stories of when bringing in investment money was a disaster. Most notably, ArsDigita went from a growing profitable business to infighting and self-destruction after taking VC investment. There were also stories of where investment worked out fine.  I think the key is to remember that the people who you take investment from are you partners for the life of the business. You can fire co-founders, divorce your wife, but investors are forever.  Choose people you feel comfortable working with and trust.

4) Every company has its Dark Days.

One question that Jessica seemed to make sure to ask every founder was if they ever “thought about quitting.”  Most responded with stories of their darkest days, when they weren’t sure they were going to make it, that they had to fire a friend or when a key deal fell through.  Fortunately, everyone interviewed stuck with it, which is why their businesses were successes and they were profiled in a book like this.

I think the biggest lesson I learned from all of the stories was just that you need to be insanely passionate about your business; it’s going to be hard, there will be setbacks and only persistence and diligence will give you a chance* at succeeding.  If you can see yourself not doing your business, then you may want to save yourself the torture of uncertainty and long hours.

If you’re starting a business or thinking about it, this is a great book to give you an idea of what it’s really like in the early days.