You need to decide this. Your cofounder will raise money based on the future needs of the company. If you're not a part of the company going forward, your cofo

You need to decide this. Your cofounder will raise money based on the future needs of the company. If you're not a part of the company going forward, your cofounder will need to make it clear to investors how he plans to move the company forward without you. If your role is critical to the company's continued existence, he may need to find a new technical cofounder, which can take quite some time. If not, he may need to raise more money so that he can hire employees or pay for contractors. If you do decide to leave it's going to be very much like breaking up... If you've been communicating well with your cofounder he probably won't be surprised, and is more likely to be mature and understanding, and start discussing all of the things that will need to be done so that you can leave without hurting the company. If you've not been communicating well, your cofounder may be completely surprised and upset, and may disagree that you leaving is the best thing for both of you. He may want nothing to do with you moving forward. It's important that you're prepared for this, and that you are able to show him exactly how you can leave the company without harming it. In other words, you'll need to make a very clear exit plan, so that it's clear that you're leaving the company, not trying to destroy it. If you're lucky he'll let you help execute the transition plan.