This is not without controversy. Some view this to mean that Christ was due to return within that generation (such as at 70 CE). Check the Greek Interlinear, using the word "Accordingly" to link the statement to the event may not be justified.
The statement by Jesus seems to suggest that some would be dead, but not all -- otherwise why say such a thing?
Paul also wrote at 1 Thess 4 (NIV):
13 Brothers, we do not want you to be ignorant about those who fall asleep, or to grieve like the rest of men, who have no hope. 14 We believe that Jesus died and rose again and so we believe that God will bring with Jesus those who have fallen asleep in him. 15 According to the Lord’s own word, we tell you that we who are still alive, who are left till the coming of the Lord, will certainly not precede those who have fallen asleep. 16 For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first. 17 After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord forever. 18 Therefore encourage each other with these words.