Natural gas often has some liquid content. The liquid may be water, hydrocarbons, or compressor oil. As the gas flows through an orifice meter is the gas being measured correctly? The measurement methods and calculations described in ANSI/API 2530 are for dry gas. Many researchers have studied the effect of entrained liquids on orifice measurement. The existing literature can provide much information about orifice flowmeter errors. This information can be used to determine the course of future orifice plate research efforts. This paper will discuss four test programs that were conducted to examine the effects of entrained liquids on orifice meter performance. The results of these programs will be discussed as well as some simple flow models. The flow models will be used to explain why research into this area has been so difficult. The flow models are not intended as a guide for flow measurement correction