In this thesis a new technique called 'attosecond angular streaking' (AAS) was applied for the first time. AAS allows to resolve ionization dynamics in the strong field regime with attosecond accuracy using only femtosecond pulses. In this regime, ionization mainly proceeds via tunneling through an energetically forbidden barrier. The process of tunneling is a fundamental and well-understood phenomenon in quantum mechanics. However, attempts to measure the 'tunneling time' have produced controversial results, partly due to the difficulty of defining a temporal operator in quantum mechanics, making it difficult to compare experiments. In this thesis, the question is addressed of whether the tunneling rate can adjust instantaneously to a changing barrier or if there is a delay between the field that defines the barrier and the corresponding tunneling ionization rate. It was found that indeed, no delay between the electric field and the corresponding tunneling rate exists within an experimental error limit of 12 attoseconds.