Climate change may affect food production and ultimately result in a food crisis. But, what phenomena does a food crisis involve and how serious must the food shortage be to qualify as a 'crisis'? To answer this, we attempt to 'define' a food crisis by applying standard microeconomic approaches and examine the history of past food crises. We first build three indices that measure the degree of food crisis, i.e., nonlinear trend deviation index, food shortage index, and food market index. Basically, each index represents a variant measure of the food market's supply shortage and/or price increase. We then use data of 182 countries from 1961 to 2009 to estimate each index and attempt to identify the occurrence of food crises for each country. All three indices indicate that two major food crises occurred in 1973 and 2008 on a global scale, while minor ones took place less uniformly for different years for different countries during the sample period. The food shortage index seems more reliable than the other two by identifying the outbreaks of food crisis more systematically.