This monograph is a compendium of information provided by leading experts in the cloud physics community who recognized the need to compile a summary of our current state of understanding of how ice forms and evolves and of our technological capability to measure the critical properties of these ice crystals. It begins with seven chapters devoted to discussion of the scientific underpinnings of cloud ice formation and evolution. Each of these chapters covers the theoretical aspects followed by the remaining challenges that hinder our basic understanding and limit our ability to successfully model the microphysical processes. The remaining chapters concern measurement methodology and challenges that include the uncertainties and limitations of current measurement techniques.