Sensors and leds. This need resistors? and how and where?

I have a project with leds and sensors, this is my first project in Arduino and I want to know if it need resistors and how and where?

I use 16-Bit I/O Expander MCP23017 for leds and multiplexer CD4051 for sensors LM35

My project.

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LEDs definitely need a resistor in series with each LED.

LM35, seems to depend on the temperature range desired. http://www.ti.com/lit/ds/symlink/lm35.pdf