Many intracellular proteins do not work on their own but rather in complex with small molecules, DNA, or other proteins. To gain a more fundamental understanding of protein interactions and their resulting functions, one requires a detailed structural model of relevant complexes. The first step in this challenge is to grow well-diffracting crystals. Three examples of protein complex crystallization will be discussed in detail below. In the first example, biophysical techniques such as fluorescence titration, isothermal titration calorimetry (ITC), and dynamic light scattering (DLS) are used to characterize the protein and assess the most suitable conditions for complex formation. The second example utilizes bioinformatic information and proteomic techniques to engineer constructs of the protein that are most favorable for crystallization. The final example uses NMR information for optimizing complex-forming conditions, which allowed the growth of better-diffracting complex crystals.