It has been 15 years since the first attosecond light pulses was reported in 2001. The generation of single attosecond light pulses still remains quite challenging today. In this talk, I  will first give a brief summary of some recent experimental progress. Then I will present our recent theoretical works that address a few fundamental issues including: (1) How accurately have the attosecond pulses been characterized? (2) How accurately the phase of an electronic wave packet can be retrieved using attosecond streaking experiments? (3) How to characterize a sub-50 as pulse in the water window region?