There can be a couple of factors at play here.  If the yellowing is widespread, you could have a lawn that is predominately Ryegrass which goes dormant, turns brown and stops growing during drought and high temperature conditions.  You could be over or under watering.  Extremes of either can cause lawn stress.  If the  areas are spotty, you could have a fungal infestation, dying grass due to pet urine, or a grub infestation.  We consider a yard to be grub-infested if a shovel of dirt yields 10 grubs per 1 square foot.  Contact our office and we will come out free of charge to diagnose the condition affecting your lawn.