The weather is the single most important factor in the mountains over which you have no control. Good weather can make a hard walk into a panoramic spectacle, or it can turn a normal days trekking into a life threatening situation. Preparation is key and thanks to advances in modern technology, it's now much easier to stay informed and make the calls that count.
Mountain weather is notoriously difficult to predict and can change at a moments notice on even the finest of spring days. While you can never be sure what the weather will do and should be prepared for any eventuality, the resources below may help you choose the best times to tackle the most important parts of your trek.
Of course you should never rule out local knowledge - an experienced local guide will have years of experience reading the subtle environmental clues unique to each region and their advice may be more reliable than anything else. Whether you join a group trek or find your own trekking guide, having local experience to call on is invaluable.