Gasifier reactors are relatively simple in construction, although the chemistry and physics involved in their operation is still not well understood. Consequently, the practice of gasification involves as much art as science. There are many papers available in the literature regarding modeling of gasifier reactors that provide an abundance of information that is helpful for reactor design. However, there are limited resources available to those individuals with the goal of successfully operating a gasifier system with biomass fuels. Therefore, the intent of this chapter is to give an overview of the gasification processes used to convert biomass fuels into a producer gas. Gasifier designs are generally categorized into three types: downdraft, updraft, and fluidized beds. Selection of a specific gasifier type will depend on the end use and the quantity of the producer gas required. The focus here will be on the practical aspects of gasification for small to medium-scale systems. Practical issues addressed in this chapter are: gasifier fuels; performance evaluation; fuel feeding; process control; and producer gas quality.