Thermodynamics studies are undertaken by the following two different approaches:
• Macroscopic approach (Macro mean big or total)
• Microscopic approach (Micro means small)
The state  or condition of the system can be completely described by measured values of pressure, temperature and volume which are called macroscopic or time-averaged variables. In the classical thermodynamics, macroscopic approach is followed. The results obtained are of sufficient accuracy and validity.
Statistical thermodynamics adopts microscopic approach. It is based on kinetic theory. The matter consists of a large number of molecules, which move, randomly in chaotic fashion. At a particular moment, each molecule has a definite position, velocity and energy. The characteristics change very frequently due to collision between molecules. The overall behavior of the matter is predicted by statistically averaging the behavior of individual molecules.
Microscopic view helps to gain deeper understanding of the laws of thermodynamics. However, it is rather complex, cumbersome and time consuming. Engineering thermodynamics analysis is macroscopic and most of the analysis is made by it.

These approaches are discussed below:

Macroscopic Approach:
• In this approach a certain quantity of matter is considered without taking into account the events occurring at molecular level. In other words this approach to thermodynamics is concerned with gross or overall behavior. This is known as classical thermodynamics.
• The analysis of macroscopic system requires simple mathematical formula.
• The value of the properties of the system are their average values. For examples consider a sample of gas in a closed container. The pressure of the gas is the average value of the pressure exerted by millions of individual molecules.
• In order to describe a system only a few properties are needed.
Microscopic Approach:
• The approach considers that the system is made up of a very large number of discrete particles known as molecules. These molecules have different velocities are energies. The values of these energies are constantly changing with time. This approach to thermodynamics, which is concerned  directly with the structure of the matter, is known as statistical thermodynamics.
• The behavior of the system is found by using statistical methods, as the number of molecules is very large. So advanced statistical and mathematical methods are needed to explain the changes in the system.
• The properties like velocity, momentum, impulse, kinetic energy and instruments cannot easily measure force of impact etc. that describe the molecule.
• Large numbers of variables are needed to describe a system. So the approach is complicated.