With Critical Dimension Uniformity requirements in optical lithography getting tighter and tighter, phenomena that previously could be ignored now need a detailed understanding and control strategy. Amongst those are the effects introduced by the finite height of the mask absorber (Mask 3D) and the finite resist height (Resist 3D). We will explain them by analyzing wafer Critical Dimension (CD) data through focus and dose and categorizing those using simple figures of merit: Best focus differences between features, Bossung tilt through focus and sidewall angle through dose. We will study the phenomena and show a methodology and gauges to discriminate between Mask 3D and Resist 3D. This will enable the end-user to judge the effects, which are highly application dependent, and choose to put the effort for future nodes either on Mask 3D or Resist 3D or both. In the second part we will focus on the solutions. The main focus is on immersion lithography we but will show the extendibility of some of the work to EUV.