Bumpy lawns are caused by mole tunnels, clumps of weeds and natural settling of the soil. The best approach to solving the problem and restoring a smooth, easy-to-mow lawn depends on the severity and underlying cause. Some lawns can be fixed without disturbing the grass; others need to be completely renovated.
Professionals fix bumpy turf on golf courses by adding thin layers of soil or sand to low areas, and you can do the same thing to your lawn. Use only high-quality fill material that is free of weed seeds, like topsoil, compost or fine sand. Don't overwhelm an area with a deep covering of fill; use several thin applications, and wait until the grass recovers enough to be mowed before applying the next layer. For home lawns, limit each application to 1/4 inch, and gently sweep it into the gaps between grass plants with a broom or lawn rake.
Bumpy areas caused by mole tunnels and hills can be flattened with a lawn roller. Unfortunately, this will not permanently solve the problem unless you also trap the moles (best done before rolling the lawn since traps require intact tunnels). A lawn roller can also flatten a newly sodded lawn that has settled as long as the soil is relatively soft. Don't use heavy rollers on new sod as they may damage the newly established plants.
A tamper consists of a handle attached to a heavy, flat metal bottom, lifted up and pounded straight down onto the ground. When a bumpy lawn is the result of a few high spots instead of a few low spots, tamping may be the simplest solution. Tamping works on new sod and established lawns with soft soil. Hard soil may be softened by watering, but don't overdo it and make a muddy mess.
Kill tufts of weeds or invading grass species like fescue or orchardgrass by applying glyphosate according to the label directions. Smooth the area with a rake or tiller after the unwanted plants die, and reseed it. If the entire lawn is bumpy and full of weeds, glyphosate can also be used to kill the entire lawn and start over; a process called renovation. Till and drag the seedbed to level it, and allow it to settle before reseeding or installing sod.