It is a stage. Basically it is at this age that they are trying to differentiate themselves as an individual separate from you (they do this again when they hit teenage years - can you tell?!?). So that is why they test boundaries and dont listen or say no to everything you ask. The best thing to do is just understand that this will happen and it will pass. It is a good time to reassess your house rules and what is appropriate and what isn't. Throw away any rules that aren't logical or necessary (make your house as toddler safe as possible to reduce to amount of rules you need to enforce) and stick to the ones that are important (especially safety rules) making sure you give a clear reason for your decision (eg. one word - safety, dangerous, fragile etc.). Follow up with clear guidelines if they insist on breaking an important rule but make sure consequences are logical. For example if they are tipping water over the floor (a favourite one n our house) - "stop because it is slippery and dangerous. If you do it once more I will take the cup. OK I am taking the cup. *cuddles or comfort during the inevitable tantrum*. Mummy loves you but you can't tip water on the floor - it is dangerous". Lots of deep breaths. LOL. You will both get through it.

A great book for understanding the development is Robin Grilles Heart to heart parenting. Very useful!