Many landlord/tenant disputes involve the security deposit. There are several important steps you should take in order to protect your deposit. The most important thing you can do is complete a written checklist to document the property condition and existing damages when you move in. You should send it to your landlord, and be sure to keep a copy for your records. You should be very detailed and thorough when completing your checklist, because if you don't make note of all existing damage, your landlord may assume that you caused the damage and charge you when you move out.
If your landlord does not return your security deposit, you have specific rights under Colorado law. Under Colorado's security deposit law, the landlord is required to send a written statement of damages, along with any security deposit refund, to the renter's last known address. The landlord has one month (unless a longer period of time, not to exceed 60 days, is stipulated in the lease) to send to your last known address the full amount of the security deposit or a written list of the damages and the amount of money needed for repairs, and whatever money remains of the deposit.
If your landlord does not comply with the security deposit law, the tenant has certain rights under Colorado law. The first step is to send a letter to the landlord, demanding the deposit. The letter should be sent by certified mail, return receipt requested. If the landlord does not respond to the letter within seven days, you can pursue court action. You may want to pursue the matter in Small Claims Court. You might want to look at a sample seven day demand letter for when you don't receive a written statement of damages, or a sample letter disputing charges for when you don't agree with the charges.
Be aware that there is a special security deposit law addressing hazardous gas leaks, discussed in the rights handout available above.