We've come a long way from iron sights, but when choosing the right riflescope for your application, here are some things to consider:

• Magnification - This is a subjective decision, but some general rules are 10x for distances less than 500 yards and 16x and higher beyond 500 yards.  The higher the optical magnification, the more glass and the more glass, the more weight and higher price.  Variable power helps with the choice and makes them suitable for many situations and conditions
• Size of Objective Lens - The objective lens is the largest and forward most lens and generally, the larger the better because it harnesses more light.  However, a larger lens requires higher mounts to keep it off the barrel and may require a higher cheek rest.  Once again, larger is heavier, bulkier and more expensive. No sense in getting more than you'll ever use so a balance between size, weight and budget for your application is what you're after.
• Reticle - There are many patterns of reticles available with the most common available is the duplex, or a simple crosshair pattern.  Mildot is the preferred reticle among tactical shooters due to the ability to range targets.  Bullet Drop Compensating reticles make use of grids that can accommodate different ranges without adjustment..
• MOA vs. MRAD - Knowing the measurement of the click values of adjustments on the scope is important.  MOA or Minute of Angle is approximately one inch every hundred yards.  MOA are commonly used with duplex and BDC reticles.  MRAD or milradians measure about 3.6 inches every 100 yards and are the unit of measure for Mildot reticles.
• Focal Plane - This is the position of the reticle within the scope. The majority of scopes use Second Focal Plane (SFP) reticles, which means when the magnification is changed, the reticle stays the same size.  First Focal Plane (FFP) reticles scale up and down with the magnification.
• Parallax Correction - Parallax is the movement of the reticle in relation to the target when the shooter moves his/her head.  It is an error the is apparent in higher magnification scopes and could result in bad shot placement.  There are usually side "focus" adjustments in high-end scopes that can eliminate the parallax.

Today there are many riflescopes available for just about any budget.  Premium scopes will cost much more and it's because of higher tolerances, durability, repeatability and additional features, as well as extended warranties.  Remember that the scope is one component of effective shooting, but the scope should be mounted properly, sighted accurately and thoroughly understood to add optimal performance to any shooting situation.