1. Introduction

2. Classification of Gas Sensing Methods

3. Performance Indicators and Gas Sensor's Stability

4. Gas Sensing Methods
4.1. Methods Based on Variation of Electrical Properties
4.2. Methods Based on Variation of Other Properties

5. Some Approaches to Improve Sensitivity and Selectivity
5.1. Introduction for Sensitivity and Improving Approaches
5.2. Introduction for Selectivity and Improving Approaches

6. Conclusions